• Mise en Place. Bring butter and eggs to room temperature (the butter should be cool but soft to the touch, not melty or greasy). Measure both sugars into one container. In another container, whisk together flour, baking powder, and salt and set aside. Sift cocoa powder. Grease a 10-cup bundt pan with non-stick spray. Set aside.
• Preheat oven to 350°F.
• Bloom cocoa powder. Scald milk in a small pot over low-medium heat on the stove, swirling the pot frequently until small bubbles form around the edges. When you tilt the pot there will be lots of active little bubbles on the bottom of the pot. You're aiming for a temperature of 170°-180°F. Do not boil the milk. Remove from heat and add the sifted cocoa powder all at once. Whisk well until no clumps remain, then add sour cream and whisk until smooth and combined. Set aside.
• Cream butter and sugar. Cut the butter into large chunks and place in the bowl of a stand mixer. Beat on medium speed to soften, about 1 minute. Scrape down the sides of the bowl and add both sugars. Beat on medium-low speed until there's no loose sugar in the bowl, then increase speed to medium and continue creaming for 3-7 minutes, pausing to scrape down the bowl and the beater at least 3 times. Properly creamed, the butter and sugar will be pale brown and have a fluffy, airy, and paste-like texture. Scrape down the sides of the bowl.
• Add eggs and vanilla. One at a time, crack an egg into a small bowl (to avoid getting shells in your cake!) then dump the egg into the mixer bowl, beating on medium speed for at least 60 seconds and scraping down the bowl again before adding the next egg. Add the vanilla along with the final egg. Scrape down the bowl and beater again at the end.
• Alternate adding dry and wet ingredients. With the mixer running on the lowest possible speed, alternate adding one third of the dry ingredients with one half of the wet ingredients, starting and ending with the dry ingredients (dry —> wet —> dry —> wet —> dry). Pause the mixer and scrape down the bowl and beater between each addition.

  Stop the mixer when the batter is mostly combined and just a few streaks of flour are visible. Use a spatula to mix in any final bits of dry ingredients from the sides of the bowl or beater, making sure to scrape and stir all the way to the bottom of the bowl where sneaky bits of butter and sugar may be hiding.
• Assemble. Spoon about half the batter into the greased bundt pan. Use a mini offset spatula to smooth out the batter, pressing it down into all the nooks and crannies of the pan so there aren't any trapped air pockets. Arrange a layer of whole Oreo cookies around the center of the pan, as close together as you can get them. Then spoon the remaining batter overtop and smooth it out into all the corners and creases of the pan. Tap the pan against the counter a few times to knock out any excess air bubbles.
• Bake. Place the bundt pan in the center of a 350°F oven for 60-65 minutes, until a toothpick or cake tester inserted in the center comes out with just a few crumbs clinging to it.
• Cool. Remove the pan to a cooling rack. Let cool 15-20 minutes, then invert the cake onto a cooling rack to finish cooling. Let cool completely before adding icing.

Icing & Topping

• Icing. Sift powdered sugar into a medum mixing bowl. Add melted butter, vanilla bean paste, and the lesser amount of whole milk. Whisk to combine. Slowly add additional milk 1 teaspoon or ½ teaspoon at a time, only as needed until you have a thick but fluid icing. The icing should fall slowly off of ithe whisk when you lift it out of the bowl and hold its shape for about 10-15 seconds seconds in the bowl before sinking into itself.
• Topping. Pipe or drizzle the icing around the top of the cake. Top the wet icing with chopped Oreo cookies, then sprinkle any remaining Oreo crumbs overtop. Allow the icing to set up for a few minutes before slicing.