Grease a baking pan with butter and dust it with cocoa powder. Set aside.
Preheat Oven
Preheat oven to 340F/170C.
Flour Mixture
In a bowl, whisk flour (1 cup, 140 grams), baking powder (2 teaspoons), and salt (1/4 teaspoon).
Egg Yolk - Cocoa Batter
Into a mixing bowl, pour egg yolks (5) and half of the granulated sugar (1/2 cup, 100 grams). Beat until mixture is thick and pale in color, about 3 minutes.Gradually beat in oil (5 tablespoons) and Vanilla Extract ( 1 teaspoon).
Sift cocoa powder into the egg yolk mixture (1/2 cup, 50 grams). Beat until combined. Scrape down the bowl.Gradually pour hot water into the egg yolk-cocoa mixture while beating at medium speed. Don't pour it into the middle of the bowl. Pour the stream of hot water onto the side of the bowl. Scrape down the bowl to make sure all is well combined.
Meringue
In a clean bowl of your mixer, beat egg whites (5) with cream of tartar (1/2 teaspoon) until soft peaks are formed. Then gradually add half of the granulated sugar (1/2 cup, 100 grams). Beat until stiff glossy peaks are formed. Don't overbeat.
Chocolate Sponge Cake Batter
Using a fine mesh strainer, sift about 1/3 of the flour mixture into the egg yolk-cocoa mixture and carefully fold the flour with a spatula, then add about 1/3 of the meringue and fold it in. Continue with the rest of the flour and meringue until no streaks are visible. Then make the last three folds; make sure you go with the spatula all the way to the bottom of the bowl. Be gentle, and don't over-mix. You are looking for a light and airy batter.
Bake
Pour chocolate batter into a prepared baking pan.Bake at 340F/170C for 25-30 minutes or until a wooden skewer inserted in the middle comes out clean. If the cake is browning too much, cover it loosely with a foil dome.Let the cake cool in the pan for 5 minutes. Then invert it on a cooling rack and let it cool completely. Wrap the cake in food-safe plastic wrap.
Notes
How to Store Chocolate Sponge Cake? - Store undecorated/unfilled cake covered in the fridge for up to 5 days until ready to use. Unfilled cake keeps for 3-4 days at room temperature. If using whipped cream, use some gelatin to stabilize it so whipped cream doesn't weep.
Why is my sponge cake dry? Most likely, the meringue was overbeaten, or the cake was baked for too long. When beating egg whites, stop the mixer a few times while whipping the meringue and test if a stiff peak is formed. When meringue is overmixed, it becomes grainy. The grainy meringue will make your cake dry. To amend a dry sponge, use simple syrup