Preheat the oven to 325˚F (170˚C).
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy, about 3-4 minutes, stopping to scrape the sides of the bowl as needed. Add eggs, one at a time, beating well after each addition. Beat in the vanilla.
In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cocoa powder. With the mixer on low speed, gradually add the flour mixture to the butter mixture alternately with the LACTAID® Chocolate Milk, beginning and ending with the flour mixture. Beat just until combined after each addition.
Spray a 10-cup Bundt pan with baking spray with flour. Carefully transfer the batter into the prepared pan. Tap the pan on a kitchen towel-lined counter a few times to settle the batter and release any air bubbles.
Bake until a wooden skewer inserted near the center comes out with a few moist crumbs, about 45-50 minutes. Let cool in the pan for exactly 10 minutes, then invert cake onto a wire rack and let cool completely outside of the pan.