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Line a round baking tray with baking paper. In a large bowl, first mix the dry ingredients - sifted flour, cocoa, sugar and baking powder - with a whisk. In a smaller bowl, using an electric mixer, mix well the wet ingredients - yoghurt, oil, eggs and add the vanilla sugar. Once the wet ingredients are fully mixed, add them to the dry ingredients in the larger bowl and stir gently, while slowly adding the brewed coffee alongside. Once the ingredients are fully combined, divide the mixture into two equal parts. Bake the first half first in a round baking tray, and the second half when it is baked. Bake at 175 °C for about 35-40 minutes. You can check whether the sponge cake is baked by sticking a toothpick into the centre of the cake. If there are pieces of dough on the toothpick, the dough is not yet baked. If the toothpick is completely dry or has very few dry bits of dough on it, the dough is baked. When the sponge cake is baked, allow it to cool for about 30 minutes before removing it from the baking tray.