Preheat your oven to 350°F (180°C) and prepare two 8-inch round cake pans by greasing them lightly with butter and lining with parchment paper.

In a stand mixer, combine cake flour, granulated sugar, dark brown sugar, baking powder, baking soda, espresso powder, and salt. Mix on low speed for about 3 minutes.

Add cubed unsalted butter to the dry ingredients. Mix on medium speed until crumbly, about 2 minutes. Add sour cream and vegetable oil, mixing until combined.

Whisk large eggs, whole milk, and vanilla extract in separate bowls. Brew hot coffee and mix with cocoa powder until smooth.

Gradually pour the egg mixture into the batter, mixing on low speed. Then, slowly add the hot coffee and cocoa mixture, stirring until smooth.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 33-36 minutes, or until a toothpick comes out clean.

Let the cakes cool in the pans for about 10 minutes, then turn onto a wire rack to cool completely.

In a bowl, beat softened unsalted butter until creamy. Gradually add cocoa powder and powdered sugar, mixing well. Pour in coffee, heavy cream, vanilla extract, and salt, then beat until fluffy.

If desired, prepare a chocolate coffee syrup by combining ingredients on the stovetop over low heat, stirring until thickened.

Once cakes are cool, place one layer on a serving plate, layer with syrup and buttercream. Top with the second layer and repeat the process. Frost the entire cake with buttercream and decorate as desired.