Grease7-inch bundt pan with butter or cooking spray. Preheat your oven to 350℉
Mix the sugar with eggs, adding one egg at a time. Beat the mixture for about 5-6 minutes then add butter and buttermilk and continue beating until light and smooth.
In a separate bowl combine flour with baking powder, add a spoonful of dry ingredients at atime. Use a hand mixer or a spatula and mix until fully combined. Spoon out one third of the batter, place in a separate bowl and add cocoa powder, mix until fully combined. Add spoonful of each batters alternatively to the bundt pan to create a marble effect. Bake for around 45 minutes.
Let the cake cool in the pan, then turn it out and leave to finish cooling on a wire rack . Break up the chocolate and melt in a saucepan on a low heat, stirring continuously until melted. Do not bring it to boil. Let it cool slightly before pouring it onto the bundt cake. Sprinkle with chopped walnuts. Enjoy!