Preheat the oven to 170ºC (340ºF) and prepare three 20 cm (8-inch) cake pans with parchment and non-stick spray.

Sift and mix the dry ingredients in a large bowl: all-purpose flour, cornstarch, granulated sugar, dark brown sugar, cocoa powder, baking powder, baking soda, and salt.

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

Gently fold the wet mixture into the dry until just combined; do not over-mix.

Divide the batter among the prepared pans and bake for 28-30 minutes, checking with a toothpick.

Cool the cakes in the pans for 10 minutes, then transfer to a wire rack to cool completely.

To make the buttercream, combine black coffee and optional espresso powder; beat room temperature butter until fluffy, then gradually add powdered sugar and coffee mixture.

Once cakes are cool, level the tops, stack with buttercream in between layers, and frost the top and sides.

Dust with cocoa powder or add chocolate shavings for decoration before serving.