I basically used a modified version of: http://allrecipes.com/recipe/chocolate-orange-marble-cake/
1 cup butter
1 1/4 cups white sugar
3 eggs
2 teaspoons vanilla extract
1 cup plain yogurt
1 3/4 cups all-purpose flour
3/4 teaspoon baking soda
3/4 teaspoon baking powder
1/4 teaspoon salt
2 tablespoons grated orange zest
3 tablespoons of cocoa powder
2 tablespoons orange extract
Orange and Black food colouring
First cream together the sugar and butter. I was using special jelly like orange food colouring which should be worked in earlier.
Divide the mixture in half and add the cocoa powder and black food colouring to one side, orange extract to the other side.
Carefully add the batter into a greased mold by spooning in a spoonful of each batter from both bowls.
It's tedious but keep working until you fill the mold. Then swirl it a bit with a knife.
Bake at 350 F for 50 min. Then let cool.