Beat the eggs at a high speed with stevia, goat yoghurt and olive oil.
Add baking soda and lemon juice. Continue beating.
Start adding almond flour, psyllium powder and coconut flour. Add in the ground vanilla bean, orange zest and stevia. Keep mixing until you get a smooth texture.
Take 1/4 of your mixture out and place it in a small mixing bowl. Add cacao to it and mix well to get a nice chocolate colour.
grease a 20 cm cake mould with some butter or coconut oil and pour in the light coloured cake batter. Now, using a spoon, add chocolate batter by creating fast circular movements over the cake so that you get a marble effect.
Bake at 180ºC for 15 minutes, then reduce the heat to 150º C and bake for another 10 minutes,