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

Brew a cup of black coffee and let it cool to room temperature.

In a large mixing bowl, sift the dry ingredients: flour, cornstarch, sugars, cocoa powder, baking powder, baking soda, and salt. Whisk until combined.

In a separate bowl, mix the wet ingredients: eggs, sour cream, oil, vanilla, and cooled coffee until smooth.

Pour wet ingredients into dry and mix gently until just combined.

Distribute the batter evenly into the prepared pans and bake for 28-30 minutes.

While cakes cool, prepare the buttercream by beating butter until creamy. Gradually add powdered sugar and coffee until fluffy.

Level cooled cake layers if needed, frost the top of one layer with buttercream, add another layer and repeat. Finish with a crumb coat and remaining buttercream.