together the butter and sugar until light and fluffy, about five minutes. slowly add in the flour, and mix to combine. don't over-mix!
in a separate bowl, whisk together the eggs, vanilla, and sour cream. slowly add into batter and mix just until combined. remove 1/3 of the batter from the bowl, and put into a separate bowl. mix in the chocolate milk mix until well combined.
place the vanilla batter into a tube pan that's been prepared with baking spray. spread out evenly in the pan. place spoonfuls of the chocolate batter all around the top of the vanilla batter. use a butter knife to swirl the chocolate into the vanilla. 