6 tablespoons unsalted butter, at room temperature
¾ cup packed dark brown sugar
2 tablespoons granulated sugar
1 large egg
1 large egg yolk
¾ cup buttermilk
1 teaspoon vanilla extract
½ cup cocoa powder
1 cup all-purpose flour
¼ teaspoon baking soda
½ teaspoon baking powder
½ teaspoon kosher salt
Frosting
2 ounces unsweetened chocolate, melted and cooled
1½ cups powdered sugar
½ cup unsalted butter, at room temperature
Pinch of kosher salt
1 tablespoons cream or whole milk (I used buttermilk here since I didn't want to waste)
½ teaspoon vanilla extract
Confetti or Rainbow Sprinkles, for topping
Instructions
Preheat oven to 350 degrees and butter a 8x8 square cake pan.
In a large bowl with an electric hand mixer, or the bowl of a stand mixer with a paddle attachment, combine butter and sugars. Mix on medium until combined.
Add in egg and egg yolk, one at a time, scraping the bowl with a spatula after each egg.
With the mixer on low, add buttermilk and vanilla extract.
In a separate bowl, sift together coco powder, flour, baking soda, baking powder, and salt.
With the mixer on low, add the dry ingredients and mix until combined.
Pour cake mixer into prepared cake pan, and spread evenly with a spatula. Bake for 20-25 minutes until a toothpick inserted into the center comes out clean. Let cool for 10 minutes in the cake pan on a cooling rack, then invert cake onto the cooling rack to cool completely before frosting.
For the frosting, in the bowl of a mixer with the whisk attachment, combine all ingredients and mix until combined. With an offset spatula (or a spoon works fine) spread the frosting on the cake, and make S-like swirls. Finish the cake by topping with rainbow sprinkles and enjoy.
Recipe by Amy vs Food at https://www.amyvsfood.com/2017/11/03/good-wont-believe-chocolate-cake/