Prepare brownie mix in a 9-inch springform pan, lined with parchment paper and sprayed with baking spray. Bake according to package directions. Remove from oven.
Cool 5 minutes in pan, then remove springform and cool on wire rack. Set aside.
For the cake layers:
Whisk together the egg whites and 1/4 cup of the milk in a small bowl until slightly combined. Set aside.
In a mixing bowl, beat butter, sugar, and vanilla for 2 minutes on medium speed, scraping down the sides of the bowl as needed. Add in dry ingredients and mix until combined. Beat in egg white mixture until fully blended. Add in remaining milk and beat for 3-4 minutes, scraping down the sides of the bowl.
Spray two 9-inch cake pans with baking spray. Pour batter into pans and bake in a 350 degree oven for 26-28 minutes. Remove and cool completely on wire rack.
For the frosting:
In a food processor, pulse the Oreo until they become a fine crumb. Set aside.
Beat butter in a large mixing bowl for about 4 minutes, until fluffy and pale in color. Add in powdered sugar and milk and beat an additional 3-5 minutes, until fluffy and smooth. Fold in 1 cup of the cookie crumbs (set aside the remaining crumbs for the garnish).
Place the brownie layer on the bottom of a cake platter. Add 1/4 of the frosting and spread evenly. Top with one of the cake layers and repeat with another 1/4 of the frosting. Top with remaining cake layer and use the last 1/2 of the frosting to cover the cake completely.
In a microwave safe bowl, combine the dark chocolate with the heavy cream. Heat over high for one minute. Remove and stir until smooth. Allow too cool for a couple minutes, then spread over the top of the frosted cake.
Immediately add the reserved cookie crumbs to the top of the ganache. Add whole Oreos if desired, for a garnish. Allow ganache to set, then serve and enjoy!
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