In a bowl, break the 2 eggs with 1 cup sugar and mix well, next add the water with 1 tablespoon vinegar and then immediately the vanilla and ammonium bicarbonate. Slowly pour the flour in, while stirring with a tablespoon - no need for a mixer.
Measure out how much oil you need at the end, before dividing the mixture in 2 - this is very important. My form is small, so keep that in mind - I have never made it in a large one. Smear the form with oil, then sprinkle with half a tablespoon of flour and shake the form well.
In a separate bowl, divide the mixture in 2, it does not matter whether they are equal in size or one is smaller. In one part, mix in the cocoa. You can garnish both parts as desired.
Pour whichever mixture you want in first, the cocoa one or the other one. Bake at 392°F (200 °C) in a preheated oven, either with only the top or only the bottom element on.
I make it regularly, if not every day, and I always have the products available. Bake for about 20-25 minutes, you can gradually reduce the temperature. When baked, invert it onto a wide tray and let it cool - then cut it with a bread knife and sprinkle with powdered sugar.