Grandma’s Oatmeal Cake

It brings back so many memories.

By

Updated February 06, 2025

baking dish with Grandma’s Oatmeal Cake, slices cut and one slice on a metal spatula with plates for serving — Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones

It may sound like a health food, but I assure you my grandma's oatmeal cake—with its tan penuche frosting—is anything but. This cake would be sitting on the counter every time we visited her. In between playing outside and ping pong, I’d enjoy a slice of oatmeal cake.

What is penuche frosting?

Penuche frosting is based on a creamy, fudge-like candy popular in New England and the American South, boasting a prominent flavor of brown sugar, and lightly touched with vanilla.

After I graduated high school, I decided to escape my small town and move across the state to live with my grandparents. That’s when I discovered that this wasn't a cake reserved for company. Grandma would make it a couple of times a month.

Every time one appeared, I would nibble on it all week long. It's the type of cake that sits on the counter, begging you to cut off a sliver every time you walk by. It’s the perfect accompaniment to an afternoon cup of coffee or tea.

It's been years since I've been to Grandma's house, but I feel transported back to those long summer days every time I take a bite of her oatmeal cake.

This oatmeal cake is very tender, with a bit of texture from the oatmeal that nicely offsets the sweet, creamy frosting. Penuche frosting tastes a lot like caramel or butterscotch; it firms as it cools and melts in your mouth with every bite.

Since this is a family favorite, I didn’t stray far from the original. The only changes I made were to use old-fashioned oats instead of quick oats and remove corn syrup from the frosting.

slice of Grandma’s Oatmeal Cake on a plate with a fork — Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones

Quick Tips on Making Oatmeal Cake:

Once the oats have absorbed the water, you can refrigerate the bowl to speed up the cooling process. However, the goal is to have the oatmeal at room temperature (not cold), so don’t leave it in the refrigerator too long.
Set a timer when boiling the frosting. If you boil it for longer than 1 minute, it can become brittle. Less, and it sticks to your teeth instead of melting in your mouth.
The frosting sets as it cools, so make sure the cake is out of the oven before whipping it. It’s best to frost the cake when it’s warm but not piping hot. The frosting melts into the cake if it’s too hot.
For a celebratory look, top the cake with sprinkles or unsweetened shredded coconut before the frosting sets.

slices of Grandma’s Oatmeal Cake on dessert plates with forks and glasses of milk — Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones

Sheet Cake Perfection

Grandma’s Oatmeal Cake

Prep Time 25 mins

Cook Time 25 mins

Total Time 50 mins

Servings 16 servings

Yield 1 (9x13-inch) cake

Cook Mode (Keep screen awake)

For the cake

1 1/2 cups boiling water
1 cup (80g) old-fashioned oats or quick oats (not instant)
Cooking spray
1 1/2 cups (180g) all-purpose flour
1 teaspoon ground cinnamon
1 teaspoon baking soda
1/2 teaspoon kosher salt
1/2 cup (112g) unsalted butter, room temperature
1 cup (200g) granulated sugar
1 cup (164g) packed light brown sugar
3 large eggs, room temperature
1 teaspoon vanilla extract

For the penuche frosting

1/4 cup (56g) unsalted butter
1 cup (164g) packed light brown sugar
1/2 cup (100g) granulated sugar
1/3 cup milk
1/8 teaspoon kosher salt

Soak the oatmeal:

In a small bowl, pour the boiling water over the oatmeal and stir to combine. Let it sit until the water is absorbed and the oatmeal is cool, about 25 minutes.

Simple Tip!

After about 15 minutes have gone by, carry on with the recipe. The oats should be cooled by the time they are added.
Preheat the oven to 350°F.

Spray a 9x13-inch cake pan with cooking spray and set aside.
Combine the dry ingredients:

Stir the flour, cinnamon, baking soda, and salt together in a medium bowl and set aside.
Combine the wet ingredients:

Mix the butter, sugar, and brown sugar in the bowl of a stand mixer on low, slowly increasing the speed to medium-high until well combined and uniform in color and texture, about 3 minutes. Stop the mixer and scrape down the bowl halfway through the mixing time. Note that the mixture doesn’t lighten in color or get completely creamy.

Turn the mixer down to low and add the eggs one at a time. Mix until incorporated, about 20 seconds before adding the next egg. Stop the mixer and scrape down the bowl, then mix on medium-low for another 20 seconds to ensure the batter is emulsified. Add the vanilla extract and mix to combine.
Add the dry ingredients and oatmeal:

Add the room-temperature oatmeal to the batter and mix on medium to combine. Once combined, turn off the mixer and add the flour and spices. Mix on low until a few streaks of flour remain, then stop the mixer and scrape the bowl a couple of times with a flexible spatula and make sure the batter is fully mixed.
Bake:

Scrape the batter into the prepared pan and spread it evenly with a flexible spatula. Bake until the sides start to pull away from the pan, the middle of the cake springs back when gently pressed, and a toothpick comes out clean, about 25 minutes.

Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones
While the cake bakes, make the frosting:

In a 2-quart saucepan, melt the butter on medium-low. Once melted, add the brown sugar, white sugar, milk, and salt and combine using a flexible spatula. Increase the heat to medium and continue stirring and scraping the sides and bottom of the pan. When bubbles reach the top of the mixture, stop stirring and boil for exactly 1 minute. Set a timer!

Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones
Whip the frosting:

Once the frosting has boiled for 1 minute, scrape it into a large heat-proof bowl. Let it cool until the cake is out of the oven (about 15 minutes). Then use a whisk to whip the frosting until it’s thick and no liquid pools in the trails left by the whisk. How cool the mixture is will determine how long it takes to whip it, but it usually only takes 1 to 2 minutes. The frosting will be light tan in color.

Simply Recipes / Photo by Julia Gartland / Food Styling by Pearl Jones
Frost the cake:

Scrape the frosting over the warm cake. Use an offset spatula to spread the frosting evenly over the top. This is not a cake for perfectionists. The frosting sets quickly and will lose its shine if you repeatedly smooth the same areas (I’ve learned this the hard way!) so spread it and walk away.

Place the cake on a cooling rack and let cool completely before cutting and serving, about 1 to 2 hours.

Cover any leftover cake in plastic wrap or store in an airtight container and leave on the counter. The cake will last for 4 to 5 days.

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Nutrition Facts (per serving)
303	Calories
10g	Fat
51g	Carbs
3g	Protein

Nutrition Facts
Servings: 16
Amount per serving
Calories	303
% Daily Value*
Total Fat 10g	13%
Saturated Fat 6g	29%
Cholesterol 58mg	19%
Sodium 152mg	7%
Total Carbohydrate 51g	19%
Dietary Fiber 1g	3%
Total Sugars 39g
Protein 3g
Vitamin C 0mg	0%
Calcium 38mg	3%
Iron 1mg	6%
Potassium 81mg	2%
*The % Daily Value (DV) tells you how much a nutrient in a food serving contributes to a daily diet. 2,000 calories a day is used for general nutrition advice.

Nutrition information is calculated using an ingredient database and should be considered an estimate. In cases where multiple ingredient alternatives are given, the first listed is calculated for nutrition. Garnishes and optional ingredients are not included.