Preheat the oven to 350. Line two baking sheets with parchment paper or spray with cooking spray.

In a large bowl or the bowl of a stand mixer, cream together the butter and sugar until light and fluffy. Add the egg and vanilla and mix well to combine. In a medium bowl, whisk together the flour, cocoa powder, salt, baking powder, baking soda, and cream of tartar. Add the dry ingredients to the wet ingredients and mix to combine. Fold in the white and dark chocolate chips.

Roll the dough into one inch balls and place 1 1/2 - 2 inches apart on the baking sheet. Sprinkle each ball of dough with a few extra white chocolate chips. Bake 8 minutes, then remove and let cool on the baking sheet for 10 minutes before removing to a cooling rack. These cookies are my favorite when they are still a little warm and the chocolate chips are slightly melted. Enjoy!