1In a small bowl, mix together the flour, baking powder, baking soda, and salt. Whisk to combine and set aside.
2Chop the Oreo cookies into small pieces and set aside.
3In a large bowl, beat the butter for 30 seconds.
4Add the sugar and the oil to the bowl and beat for 2 minutes until the mixture is creamy and fluffy.
5Add the sour cream and the vanilla to the bowl and mix to combine.
6Add 1/3 of the milk to the bowl and mix on low to combine.
7Then add 1/3 of the dry ingredient mixture to the bowl and mix to combine on low.
8We're going to alternate the milk and the dry ingredients by adding them to the bowl in thirds.
9Add the remaining milk and stir to combine and finish with the rest of the flour and stir on low.
10Set the bowl aside.
11With clean beaters, whip the egg whites for about 5 minutes until they have stiff peaks.
12Add half of the whipped egg whites to the cake batter bowl. Fold with a spatula carefully until combined.
13Add the remaining egg white mixture and add the chopped Oreos along with it and fold gently with a spatula to combine until the batter is incorporated.
14Divide the batter evenly between 3 8-inch cake pans lined with parchment paper and greased.
15Bake the cake layers in a preheated oven for 35 minutes or until a toothpick comes off clean when inserted in the cake.
16Remove the cakes from the oven and let them cool down completely before frosting.
17Place the chopped white chocolate in a bowl along with a 1/4 cup of heavy cream. And then microwave it until the chocolate has melted with the cream.
18Let the white chocolate and cream mixture cool down completely.
19Beat the cream cheese with a mixer for 45 seconds.
20Add the cold white chocolate and heavy cream mixture to the bowl and beat for 30 seconds to combine.
21In a separate bowl, beat the remaining 3/4 cup of heavy cream for 2 to 3 minutes until stiff peaks form.
22Add the whipped cream and vanilla to the bowl with the cream cheese and white chocolate mixture and fold to combine.
23Add the chopped Oreos and mix.
24Beat the butter with a mixer for 30 seconds.
25Add the cream cheese and beat with the butter for 1 more minute.
26Add the powdered sugar and the salt and mix on low until incorporated.
27Raise the speed to medium high and beat for 2 minutes until the mixture is creamy.
28Add the vanilla and a 1/2 a cup of Oreo crumbs and mix until incorporated.
29Place one cake layer on top of a serving plate.
30Brush with some sugar syrup if desired.
31Pipe a ring of cookies and cream frosting around the edges of the cake layer.
32Spoon some of the cookies and cream mousse in the center. Use a spatula to spread it out evenly.
33Top with another cake layer.
34Brush with the sugar syrup again if desired.
35Pipe a ring of cookies and cream frosting around the edges of the layer.
36Spoon more of the cookies and cream mousse in the center. Spread with a spatula.
37Top with the remaining cake layer.
38Spread the cookies and cream frosting on top of the cake and on the sides.
39Use a bench scraper or a cake scraper to smooth out the sides and an offset spatula to smooth out the top.
40Then use your hands to attach the remaining 1/2 cup of Oreo crumbs to the side of the cake on the bottom.
41Place the cake in the fridge for 30 minutes while you make the ganache drip.
42Heat up the heavy cream until hot and pour over the chopped chocolate in a bowl.
43Whisk until the chocolate has melted entirely.
44Let the ganache cool down for about 10 to 15 minutes before pouring on top of the cake.
45You can use a spoon or a piping bag to pour the ganache on top of the cake.
46I like to place the ganache in a piping bag and snip a small hole with scissors on the piping bag. And then I place the piping bag on the edge of the cake and I spin the cake plate around as I let the ganache drip. And then you can pour ganache on top of the cake and use a spatula to smooth it out.
47Place the cake in the fridge for 20 minutes so the ganache has a chance to set.
48Pipe the remaining frosting on top of the cake. I use the tip 4B to do this.
49Then you can decorate the cake with halved Oreo cookies and more cookie crumbs if desired.