Preheat the oven to 350°F. Grease a mini loaf tin (5 3/4 x 3 x 2") with non-stick cooking spray and set aside.

In a medium bowl, using a hand mixer, cream together the butter and sugar until fluffy and lighter in color. Then mix in the egg and vanilla.

Mix in about half of the flour mixture and the baking powder. Then mix in the 3 tbsp. of milk, then the remaining flour, until just combined.

Move 2 heaping tbsp. of the batter to a separate bowl...this will be your chocolate batter. Pour the remaining batter into the loaf tin.

To the additional 2 tbsp. of batter, stir in the remaining 1 tsp. of milk and the cocoa powder. Once well blended, place dollops of the chocolate batter over the top of the other batter and swirl it together with either a toothpick or knife.

Bake for 35-40 minutes, or until a toothpick inserted into the center comes out mostly clean with just a few moist crumbs. You may want to cover you loaf tin loosely with foil after the 25 minute mark. This isn't called for in the original recipe, but I think mine browned a bit too much around the edges.

Once done, place the muffin tin on a wire rack to cool completely before removing from the pan. Slice and enjoy!