Preheat oven to 325 degrees F.
First prepare chocolate sauce for that you need to cook together 4 tbsp sugar, the cocoa powder, water and corn syrup in a small saucepan until sugar completely dissolve in the syrup.
After that cool it to room temperature add baking soda in it and then set aside.
Now in mixing bowl, beat butter until become smooth then gradually add rest of the sugar and beat again to make thick fluffy mixture.
Sift together dry ingredients including flour, baking powder and salt.
Add eggs one by one and beat until eggs are well blended in the mixture and a light fluffy batter would form.
Now alternately add dry mixture and coconut milk to the batter and slow down the beater speed. When all the ingredients incorporate well with each other mix 1 cup of this batter into the cocoa syrup.
Fold the coconut flakes and extract into the remaining batter and then pour both of the batter in such a way that a marble look will form after baking i.e. first pour half of the coconut batter then sprinkle some coco batter in circular pattern and then again make layer of both these batters in the same way and then place in preheated oven for 80-90 mins until become done.