Preheat your oven to 350 degrees. Spray a 9×13 metal pan on the bottom, line with parchment paper, and spray again. If you need to use metal binder clips to hold down the sides, remember to use metal, not plastic. Set aside
Place the Oreo cookies in a food processor or blender and pulse until the Oreos are finely ground.
In a medium-sized bowl, combine the Oreo crumbs, sugar and melted butter. Stir until all the Oreo crumbs are moistened by the melted butter. You should be able to scoop it by hand and have it all stick together.
Pour the mixture into the prepped pan and press down firmly with your hand or the back of a flat measuring cup. Evenly spread across the pan!
Bake for 12 minutes. Let cool while you make the cake batter.
FOR THE CAKE BATTER
Lower your oven to 325F. In a medium sized bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, oil, and sugar on medium speed until fluffy and smooth, about 3 minutes. With a rubber spatula, scrape the bottom and sides of the bowl.
Start adding the egg whites, one at a time, beating on medium speed after each addition. I usually beat each egg white/whole egg for about 30 seconds. Scrape the bowl after each addition. Once all the eggs are added, beat for 30 more seconds.
Add the sour cream and both vanilla’s. Beat on medium low speed for a minute. If the mixture looks clumpy, don’t worry, once you add the flour mixture it will smooth out.
Turn the mixer on low and add ⅓ of the flour and ½ of the buttermilk to the butter mixture and mix ON LOW until almost incorporated. Add another ⅓ of the flour and the rest of the buttermilk. Mix just until a few streaks of flour remain, then add the rest of the flour mixture. Mix just until the flour streaks are mostly gone.
With a rubber spatula fold in the crushed oreos. Scrape the bottom and the sides of the bowl.
Pour into the prepped pan with oreo crust and bake for 40-43, or until a toothpick comes out with just a few moist crumbs on it or until it starts to get a little golden brown on top.
Remove the cake from the oven and let it cool completely cool in the pan.
While the cake is cooling make the caramel.
For the Caramel Sauce:
In a medium-sized saucepan, combine water, granulated sugar, and corn syrup. Gently mix together before turning on the heat!
Over medium heat, bring the sugar mixture to a boil. Initially, bubbles will be fast and small, but as it caramelizes, they will slow down and become larger. Do not stir during this process, as it may cause crystallization.
Let the mixture boil until it turns golden brown. This may take a few minutes, so keep a close eye on it to avoid burning.
Heat the cream in the microwave for 1 minute.
Once the caramel achieves a golden brown color, remove it from heat and add the warm cream. Stir until the caramel and cream are thoroughly combined.
Transfer the caramel mixture to a separate bowl, then add vanilla, salt, and butter. Let it sit until the butter melts, and then stir until well incorporated.
FOR THE OREO BUTTERCREAM:
In a stand mixer fitted with a paddle attachment, cream the butter on medium-high speed for about two minutes.
With the mixer on low speed, add all of the sifted powdered sugar at once and mix on low just until incorporated. With a rubber spatula, scrape down the sides of the bowl.
Add the heavy cream, salt and vanilla, increase the speed to medium-high and beat the frosting for five minutes. It should get lighter in texture and color.
With the rubber spatula, gently stir in the oreo cookie crumbs until you start seeing gray streaks.
Assembly:
You can leave the cake in the pan. Once it’s completely cool, frost the top of the cake with the oreo frosting. Evenly spread it across the top.
Add dollops of caramel sauce to the frosted cake (you’ll only need about half of the caramel; serve the remaining half on the side with each slice).