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

In a large mixing bowl, whisk together the all-purpose flour, cornstarch, granulated sugar, dark brown sugar, unsweetened Dutch cocoa powder, baking powder, baking soda, and salt.

In another bowl, mix the cooled black coffee, vegetable oil, sour cream, eggs, and vanilla extract until smooth.

Slowly pour the wet mixture into the dry ingredients, stirring gently until smooth and glossy.

Evenly divide the batter among the prepared cake pans and bake for 28-30 minutes.

Prepare the creamy buttercream by beating the softened butter until fluffy, then gradually adding the sifted powdered sugar, vanilla extract, and cooled black coffee.

Once the cake layers are completely cooled, trim the tops, layer them on a platter, and generously frost with buttercream.

Decorate with chocolate shavings, coffee beans, or a sprinkle of cocoa powder, then let it sit for a few minutes before slicing.