Preheat the oven to 350 F.
Grease a 9-inch springform pan.
Sift the flour and cocoa powder together. Set aside.
Place the eggs, sugar, and vanilla in a stand mixer fitted with a whisk attachment.
Beat on high speed for 10-12 minutes.
Turn off mixer. Sprinkle half of the flour/cocoa powder mix over top.
Gently fold in by hand. Repeat with remaining flour/cocoa powder mix.
Add the butter and fold in by hand.
Pour cake batter intothe prepared pan.
Bake for 15 -20minutes.
Cool the sponge cakein the pan before unmolding it to a wire rack.
