Preheat the oven to 350°F. Butter and flour three 6-inch pans and line the bottom with parchment paper. (I use soaked fabric cake strips on my pans for flat, softer cake layers.)

Sift the flour, sugar, baking soda, baking powder, and salt together in a large bowl.

In a stand mixer fitted with a paddle attachment, beat the butter on medium speed until light and fluffy, about 1 minute. Add the dry ingredients into the butter, and mix on low speed until well incorporated. Stop and scrape the bowl at least once during mixing. (The mixture should look crumbly.)

Combine the egg whites, sour cream, milk, and vanilla together in a medium bowl. Whisk until well combined.

With the mixer on low speed, add the wet ingredients to the butter and flour mixture and beat until combined and fluffy, about 30 seconds. Fold in the crushed Oreos until just combined. Divide the mixture evenly into the prepared cake pans.

Bake for 25 to 30 minutes or until the centers are springy to the touch. Let the cakes cool completely in the pans, then remove them.

Beat the butter on medium speed using a hand mixer or stand mixer fitted with the paddle attachment, until light and fluffy, about 2 minutes.

With the mixer on low speed, gradually add the confectioners' sugar and cream. Continue mixing until smooth and fluffy.

Reserve about a cup of the buttercream for the white dollops. Place in a piping bag fitted with an open star tip (#869 tip). Mix the crushed Oreos into the remaining buttercream.

Heat the cream in the microwave until it is steaming, about 1 minute.

Combine the chocolate and hot cream together in a small microwave safe bowl. Microwave at 50% power in 20-second intervals, stirring between each one, until smooth and combined (about 40 seconds). Set aside to cool slightly.

Spread a layer of Oreo buttercream on top of one cake layer (a heaping ⅓ cup), and add a swirl of chocolate ganache. (Don't go overboard with the ganache as it will overpower the other flavors.) Top with another cake layer, buttercream and ganache.

Add the final cake layer, then coat all over the outside of the cake with buttercream. (You can add a skirt of crushed Oreos at the bottom of the cake, if desired, by pressing Oreo cookie crumbs to the bottom of the cake with your hand.)