Set the oven to 175°C (350°F). Grease two 9-inch round cake pans, sprinkle a little flour over them, or line them with parchment paper. This makes it easier to remove the cakes later.

In a bowl, combine the flour, cocoa powder, baking soda, and salt. Give them a good sift so everything is evenly mixed. Set it aside for later.

In a large bowl, whisk together the eggs and sugar until the texture thickens a little. Add the sour cream, vegetable oil, and brewed coffee. Stir until everything is smooth and uniform.

Slowly fold the dry ingredients into the wet mixture with a spatula. Stop once everything is combined. You want the batter thick and smooth don’t over mix or the cake can turn dense.

Divide the batter between the pans evenly. Bake for about 35–40 minutes. Test by inserting a toothpick in the center it should come out clean. Let the cakes sit in their pans for 10 minutes, then gently move them to a wire rack to cool completely.

Melt semisweet and unsweetened chocolate together, either in a double boiler or carefully in the microwave. Let it cool a bit. Meanwhile, in a clean bowl, whisk the egg whites and sugar over a double boiler until the sugar dissolves and the mixture reaches around 70°C (160°F). Whip it until stiff peaks form and the mixture is slightly cooled.

Beat the butter until creamy. Slowly add the whipped egg whites. Fold in the melted chocolate and vanilla extract. The frosting should be silky and smooth.

Place one cooled cake layer on your serving plate. Spread a thick layer of buttercream over it. Add the second layer and cover the entire cake with the remaining frosting. Smooth it with an offset spatula. Chill the cake a little before slicing it helps the pieces hold their shape.