Preheat your oven to 170ºC (340ºF) and line three 20 cm (8-inch) cake pans with parchment paper. Grease the sides lightly.

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

In a separate bowl, whisk together the black coffee, vegetable oil, sour cream, eggs, and vanilla extract until smooth.

Gradually pour the wet mixture into the dry ingredients, stirring gently until just combined.

Divide the batter evenly among the prepared cake pans and bake for 28-30 minutes. Allow to cool in the pans for about 10 minutes before transferring to wire racks.

To prepare the buttercream, beat the softened butter until creamy, then mix in the powdered sugar, vanilla extract, and espresso powder until fluffy.

Once the cake layers are cool, level the tops if necessary. Spread coffee buttercream between each layer before applying a crumb coat and the final layer of frosting.