A small-batch dark chocolate birthday snack cake topped with a light, fluffy chocolate American buttercream and rainbow sprinkles—perfect for small celebrations or an easy midweek treat.
This one-bowl chocolate cake is ideal when you want something rich and chocolatey but don’t need to feed a crowd. It bakes in a 9×9 pan, produces tender, deeply flavored crumb, and comes together quickly. The hot coffee in the batter intensifies the chocolate without tasting like coffee, and the simple chocolate American buttercream is soft, silky, and perfect for swirls and sprinkles.
I originally made this for a second birthday—my family loved it so much it disappeared fast—so I tested it again to make sure the instructions were clear. The recipe is forgiving, easy to scale slightly, and stores well.
How to Make Dark Chocolate Birthday Snack Cake
Bake the Cake
Preheat your oven to 350°F. Line a 9×9 baking pan with parchment paper, leaving an overhang so you can lift the cake out easily after it cools. Lightly grease the pan or parchment if desired. In a large bowl, whisk together the dry ingredients: all-purpose flour, granulated sugar, dark cocoa powder, baking soda, baking powder, and salt.
Add the wet ingredients: room-temperature buttermilk, vegetable oil, egg, egg yolk, and vanilla. Whisk until mostly smooth. While whisking, slowly pour in the hot coffee and continue to mix until the batter is even and no large flour pockets remain. The coffee deepens the chocolate flavor—use decaf or hot water if you prefer, but coffee is recommended for the best chocolate intensity.
Pour the batter into the prepared pan and bake 28–30 minutes, or until a toothpick inserted near the center comes out mostly clean with a few moist crumbs. Remove from the oven and let cool completely before frosting.
Make the Chocolate American Buttercream
In a stand mixer fitted with the paddle (or using a hand mixer), beat room-temperature unsalted butter on medium speed for 5–7 minutes until pale and creamy, stopping to scrape the bowl as needed. Add salt and vanilla and mix to combine.
With the mixer on low, add powdered sugar in two additions (about 2 cups at a time), mixing for a few minutes after each addition until smooth. Add cocoa powder and mix until fully incorporated. Add heavy cream 1 tablespoon at a time until you reach a soft, spreadable consistency—about 2–4 tablespoons, depending on your preference. Beat an additional 1–2 minutes for a light, fluffy texture.
Spread the buttercream over the cooled cake with an offset spatula. Use the back of a spoon to create gentle swirls, then finish with rainbow sprinkles for a festive touch.
Ingredients
For the Dark Chocolate Cake
- 1 1/2 cups all-purpose flour
- 1 cup granulated sugar
- 1/4 cup + 2 tablespoons dark cocoa powder
- 1 teaspoon baking soda
- 1/2 teaspoon baking powder
- 1/2 teaspoon salt
- 1/2 cup buttermilk, room temperature
- 1/4 cup vegetable oil
- 1 large egg + 1 egg yolk, room temperature
- 2 teaspoons pure vanilla extract
- 1/2 cup hot coffee
For the Chocolate American Buttercream
- 3/4 cup unsalted butter, room temperature (1 1/2 sticks)
- 1/2 teaspoon salt
- 1 teaspoon pure vanilla extract
- 4 cups powdered sugar
- 1/2 cup cocoa powder
- 2–4 tablespoons heavy cream, room temperature
Step-by-Step Instructions
- Preheat oven to 350°F and line a 9×9 pan with parchment.
- Whisk together flour, sugar, cocoa, baking soda, baking powder, and salt.
- Add buttermilk and oil and whisk to combine.
- Whisk in egg, egg yolk, and vanilla.
- Slowly whisk in hot coffee until the batter is smooth.
- Pour batter into the prepared pan and bake 28–30 minutes. Cool completely.
- Beat butter for 5–7 minutes until creamy; add salt and vanilla.
- Add powdered sugar in two additions, mixing after each; then add cocoa powder.
- Stir in heavy cream 1 tablespoon at a time until the frosting is soft and spreadable.
- Frost the cooled cake, create swirls with a spoon, and top with sprinkles.
Notes and Tips
Storage: Store the frosted cake at room temperature for up to 2 days or refrigerate for up to one week. If refrigerated, bring to room temperature before serving for the best texture and flavor.
Freezing: You can freeze the unfrosted cake layer, wrapped tightly, for up to 3 months. Thaw in the refrigerator overnight, then bring to room temperature and frost. The frosted cake can be chilled and then briefly frozen for longer storage, but for best results freeze unfrosted.
Variations and substitutions: Use decaf coffee or hot water if you prefer no caffeine—coffee enhances chocolate but won’t make the cake taste like coffee. For a dairy-free buttercream, substitute a vegan butter and a plant-based cream alternative, keeping in mind texture may vary.
Serving suggestions: Slice into small squares for a snack-size treat or cut larger pieces for dessert. The cake is great with fresh berries, a scoop of vanilla ice cream, or simply on its own with a cup of tea or coffee.
If you enjoyed this recipe, try other favorite chocolate recipes or adapt this small-batch formula to make cupcakes or layer cakes.