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

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

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

Gradually pour wet mixture into dry ingredients and gently stir until just combined.

Divide batter evenly among prepared pans and smooth the tops. Bake for 28-30 minutes, until a cake tester comes out clean.

Allow cakes to cool in the pans for 5 minutes before transferring to wire racks to cool completely.

To make the frosting, beat softened butter until light and fluffy. Gradually add powdered sugar and mix at low speed until incorporated.

Stir in black coffee (or espresso powder) and vanilla extract. Beat until smooth and creamy.

Level the tops of the cooled cake layers if necessary. Place the first layer on a serving plate and spread a layer of buttercream on top.

Repeat with the second layer, topping with a final layer of the third cake. Frost the sides and top with remaining buttercream.

Garnish with chocolate shavings or a dusting of cocoa powder if desired.