Preheat your oven to 170ºC (340ºF) and prepare three 20 cm (8-inch) round cake pans by greasing and lining with parchment paper.

Brew a strong cup of black coffee and let it cool to room temperature.

In a large mixing bowl, sift together all-purpose flour, cornstarch, granulated sugar, dark brown sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.

In another bowl, whisk together cooled coffee, vegetable oil, sour cream, eggs, and vanilla extract until smooth.

Gradually pour the wet mixture into the dry ingredients and gently fold them together until just combined.

Distribute the batter evenly among the prepared cake pans and bake for 28-30 minutes. Let the cakes cool in the pans for about 10 minutes before transferring to a wire rack.

To make the buttercream, beat softened butter until light and fluffy, then gradually add powdered sugar, mixing well. Add cooled coffee and vanilla, whipping until smooth.

Level the tops of each cake layer, place one on a serving plate, spread with buttercream, and repeat with the second layer. Crumb coat the sides and finish frosting the top and sides of the cake.