Preheat the oven to 350°F. Grease and flour a 9x5 inch loaf pan and set aside.
In a medium bowl, mix together the flour, cocoa powder, baking powder, baking soda, and salt.
In a large bowl, beat the butter and sugar until light and fluffy. Add the eggs one at a time, mixing well, then stir in the vanilla.
Alternate adding the dry ingredients and buttermilk to the batter, beginning and ending with the dry ingredients. Mix just until combined, then fold in the chocolate chips.
Pour the batter into the prepared pan and bake for 55 to 65 minutes, until a toothpick inserted in the center comes out clean.
Cool the loaf for 10 minutes in the pan, then transfer to a wire rack to cool completely.
Melt the dark chocolate chips with half of the butter until smooth. Melt the white cooking chocolate with the remaining butter in a separate pan.
Drizzle both chocolates over the cooled loaf. Let set before slicing and serving.
