Beat the eggs and sugar with a mixer, add the yoghurt with the baking soda in it, the oil and beat well.
Add the flour in batches with the baking powder and vanilla. Pour 2/3 of the finished mixture into a lightly floured and greased Teflon tray or cake pan. Stir the rest of the mixture with 1 tablespoon cocoa or 1 packet of 3 in 1 coffee.
Pour the cocoa mixture randomly on the rest. Bake in a moderately heated 356°F (180 °C) oven for about 45 minutes.
Cool and invert into a suitable dish. The decoration is optional.