• Preheat the oven to 350°F. Grease and line three 8-inch cake pans with parchment paper.
• Sift the all-purpose flour, cornstarch, baking powder and salt into large bowl.
• Beat the butter and sugar in a large mixing bowl until light and fluffy, about 3-4 minutes.
• Add the egg whites one at a time, mixing well after each. Stir in the vanilla extract.
• Alternate adding the dry ingredients and the milk, starting and ending with the dry ingredients. Stir in the sour cream until just combined.
• Fold in the chopped Oreos, being careful not to overmix.
• Divide the batter evenly between the three prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
• Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely. Place in the fridge to chill for 30 minutes before assembling.

Make the Buttercream Frosting

• Beat the butter until smooth and very light in color, about 5 minutes on medium-high speed.
• Add the powdered sugar gradually, mixing well after each addition.
• Stir in the vanilla extract and salt, and add the heavy cream one tablespoon at a time, beating well between each addition. (I used this vanilla because it’s clear and helps keep the buttercream white)
• Once all the cream is added, continue to beat the frosting for 3 to 5 minutes until it’s smooth, fluffy, and easy to spread.

Make the Oreo Buttercream for Piping

• Take 1 cup of the prepared buttercream and mix in the finely crushed Oreos until well combined. Set aside for piping.

Assemble the Cake

• Place the first cake layer on a serving plate and spread an even layer of buttercream on top.
• Repeat with the remaining layers, then frost the entire cake with the plain buttercream.
• Press crushed Oreos around the bottom edge of the cake.
• Pipe swirls of Oreo buttercream on top using a large star tip. ( I used tip 195 from this set. )
• Sprinkle additional crushed Oreos over the top and place mini Oreos between the swirls.
• Chill the cake for at least 1 hour before slicing for clean layers.