The best chocolate sponge cake recipe

The Best Chocolate Sponge Cake Recipe
Ingredients 
6 large eggs
3/4 cup white granulated sugar
1 tsp vanilla extract
1 1/3 cup all-purpose flour
1/3 cup cocoa powder, I used Hershey’s Special Dark
1 tsp baking powder
1/2 cup unsalted butter, melted; optional
1 tablespoon instant coffee
1 tablespoon hot water
For Chocolate Cake (as seen in video):
1 recipe Vanilla Custard Buttercream, optional
1/2 cup blueberry preserves
1 recipe chocolate ganache, optional
Instructions
Preheat the oven to 350F/177C. Prepare two, 8-inch/20-cm cake rounds by lining the bottom with parchment paper; do not grease the sides. This recipe will also work well in a 13×19-inch jelly roll pan for Swiss roll cakes.

Place the eggs, sugar and vanilla extract into a stand mixer bowl, then whisk on high speed for 7 to 9 minutes. The eggs need to be thick, pale in color and super fluffy. In a separate bowl, combine the dry ingredients: flour, baking powder and cocoa powder. Sift the dry ingredients into the egg mixture in small increments, folding gently but thoroughly after each addition.
In a small ramekin, dissolve the instant coffee with the hot water. Pour the dissolved coffee and melted butter last into the batter. Fold and mix gently for approximately 30 seconds, just until the butter is mixed into the batter. (Adding the butter is optional)

Divide the batter evenly between the two baking pans. Bake in the preheated oven for 18 to 20 minutes, until the tops are set. They will spring back when tapped gently when done. Remove the cakes from the oven and place onto a cooling rack. Immediately, run a knife along the edge of the pan and cake to release the cake from the pan. Cool the layers completely on a wire rack.
Once the layers are cooled, remove the parchment paper and use a long serrated knife to split the layers in half, if desired. Use choice of fillings and frostings to fill and garnish.