Preheat oven to 350
Spray a loaf pan with cooking spray and set aside.
Beat the butter and sugar until light a fluffy. Add vanilla and eggs and beat until combined.
In a separate bowl, mix together flour and baking powder.
Add the flour mixture and milk until well combined.
Remove 1 ½ cups of the mixture and add the cocoa. Mix until combined.
Alternate filling the loaf pan with spoonfuls of the regular batter and the cocoa mixture. Using a knife pull through the batter first the length of the pan a couple of times and then the width of the pan. The best swirling effect comes from pulling the knife through the batter only a few times. Bake in preheated oven for 50 - 60 minutes or until golden and a toothpick inserted in the center of the loaf comes out clean.
