Start by blooming the cocoa powder in the hot coffee. Mix them together in a separate bowl and let it sit for a few minutes if you like.
In a mixing bowl, combine the room temperature sour cream, whole milk, eggs, vegetable oil, and vanilla extract.
Add the bloomed cocoa coffee mixture to the wet ingredients and mix everything together until well combined.
In another bowl, whisk together the cake flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.
Gradually incorporate the dry ingredients into the wet mixture, stirring until just combined.
Pour the batter into an 8x8 square cake pan.
Bake the cake until a toothpick inserted in the center comes out clean.
Once baked, take the cake out of the pan and allow it to cool completely before frosting.
To prepare the buttercream, beat the unsalted butter until creamy.
Gradually add the powdered sugar, cocoa powder, vanilla extract, and heavy cream, mixing until the mixture is smooth and fluffy.
Frost the cooled cake with the buttercream and add sprinkles if you wish.
