Preheat the oven to 350°F. Butter and flour three 6-inch pans.

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

In a standing mixture fitted with a paddle attachment, beat the butter on medium speed until light and fluffy.

Add the dry ingredients into the butter, and mix on low speed until well incorporated, scrape the bowl at least once.

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 mixture and beat until combined and fluffy, about 1 minute. 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.

Melt chocolate and cream together in a small saucepan over low heat, whisking well, until smooth and combined. Set aside.

Spread a layer of Oreo buttercream on top of one cake layer, and add a swirl of chocolate ganache. Top with another cake layer, buttercream and ganache.

Add the final cake layer, then coat all over the outside of the cake with buttercream.

Add a skirt of crushed Oreos at the bottom of the cake, if desired.

Pour the ganache on top and allow it to drip down the side. Smooth the top with an offset spatula.

Pipe white buttercream dollops on top with an 869 tip. Chill the cake for 1 hour before slicing.