Preheat oven to 180 degrees. Grease and flour a large loaf pan.

Cream butter, sugar and vanilla extract till fluffy, around 8-10 minutes. Add one egg at a time and beat at each addition.

Sift together the dry ingredients into a bowl. Add half the mixture and half the buttermilk and beat to combine.

Add the remaining dry ingredients and buttermilk and beat till just combined.

Take out a little less than half of the mixture into another bowl. Fold in the cocoa powder well.

Into the prepared pan, alternate the batter to form a marble look. Bake for around 40 minutes or till a skewer comes out clean.

Cool in pan for five minutes, and then flip onto a rack to cool completely. Slice and serve.