Cream 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. Tap the pan several times on the counter to distribute batter and eliminate any gaps.
Bake for about 30 minutes, or until a toothpick comes out with moist crumbs. Let cool in pan for 10 minutes, and then invert onto serving tray.