Preheat the oven to 350F (180C). Grease and line with parchment paper two 12×16 inch (30x40cm) baking sheets. Set aside.
Separate egg whites from yolks. Add ¾ cup (150g) sugar over the yolks and mix until creamy. Add vanilla and mix to combine.
In a medium bowl whisk flour with cocoa powder, baking powder and salt and incorporate into the yolk mixture.
Whip whites until foamy. Gradually add ¼ cup (50g) sugar and continue whipping until stiff peaks form.
Gently and gradually fold the whites into the yolks mixture.
Divide batter evenly into the prepared baking sheets. Spread the batter evenly and bake for about 8-10 minutes or until a toothpick inserted into the center comes out clean.
Sprinkle the sponge cakes with powdered sugar and cover them with parchment paper. This helps the sponge cake not stick. Flip the cake over the paper. Remove the parchment paper it was baked with. Let cool completely.