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

In a large bowl, sift and combine the all-purpose flour, cornstarch, granulated sugar, dark brown sugar, Dutch cocoa powder, baking powder, baking soda, and salt. Whisk until well blended.

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

Gradually pour the wet mixture into the dry ingredients, stirring gently until the batter is smooth.

Divide the batter evenly among the prepared cake pans and bake for 28 to 30 minutes.

Allow the layers to cool in the pans for about 10 minutes before transferring them to wire racks.

Prepare the buttercream by beating softened butter until creamy, then add powdered sugar and mix at low speed. Incorporate vanilla extract and optional espresso powder.

Once the cake layers are cool, assemble by leveling tops if necessary, spreading buttercream between layers, and applying a crumb coat.

After the crumb coat has set for about 15 minutes, apply a final thick layer of buttercream and smooth it out.