1.    Preheat oven to 160 degrees C.

2.    Cream butter and yogurt till light and fluffy.

3.    In a separate bowl, combine Optima and eggs. Beat on medium speed for 8 mins. 

4.    Add butter into the Optima mixture and beat till combined.

5.    Remove 150g of the cake batter and sift in 1.5tbsp of cocoa powder. Mix well using a spatula.

6.    Add the cake batters into a cake tin and swirl with a skewer. 

7.  Bake for 35 minutes or till a skewer test comes out clean.