Preheat oven to 350°F. Line three or three 8 inch round pans with parchment rounds, and grease with non-stick cooking spray.

Mix together all dry ingredients (flour, baking powder, sugar, and salt) in a stand mixer with a paddle attachment or hand mixer until fully combined.

Mix chunks of room-temperature butter slowly into the dry mix, on a low speed. Continue to mix until no large chunks of butter remain, and the mixture becomes crumbly.

Pour in egg whites, and mix on low until just incorporated. Mix in the buttermilk in two installments, on a low speed.

Add in vanilla extract and oil, and mix at a low speed until fully incorporated.

Scrape down the sides of the bowl with a spatula, then beat on low speed for about 15 seconds (the key is to mix until the ingredients are JUST incorporated, so that the batter isn't over-mixed). 

Fold the rainbow sprinkles sprinkles and crushed oreos into the cake batter using a rubber spatula, just they've evenly distributed.

Divide batter evenly between the prepared cake pans.

Bake for 34-35 minutes, or until a toothpick comes out clean. Allow the pans to cool for 10 minutes, then run an offset spatula around perimeter of the pan to separate the cake from the pan.

Place cake layers into the freezer for 45 minutes, to accelerate the cooling process. Once the layers are fully cooled, carefully flip the pans and remove the layers from the pans.

Use a serrated knife to level the tops of the layers, and then frost as desired.

While the cake layers bake and cool, make the vanilla buttercream frosting.

Beat the butter on a medium speed for 30 seconds with a paddle attachment or hand mixer, until smooth.

Mix in the vanilla extract and salt on a low speed.

Slowly add in the powdered sugar, 1 cup at a time.

Alternate with the heavy cream and caramel, which will help thin the frosting. You can use homemade caramel sauce (I linked my recipe above), or you can use store bought caramel sauce.

Beat on low until the ingredients are fully incorporated, and the desired consistency is reached. 

Once fully mixed, beat on low until for a couple extra minutes to make the frosting super smooth and push any extra air out of the frosting.

If the frosting is too thick, add in additional cream (1 Tbsp at a time). If the frosting is too soft, add in more powdered sugar (quarter of a cup at a time).

Cover with plastic wrap to prevent crusting, and set aside.

Stack and frost cake layers on a greaseproof cake board, using a dab of frosting to help stick the first cake layer to the board.

Add an even layer of buttercream between each cake layer with a large offset spatula.

Spread a thin coat of frosting around the the cake, to fully cover the cake layers. Smooth using a bench scraper, then chill the cake in the fridge (20 minutes) or freezer (10 minutes) until the frosting is firm to the touch.

Add a second, thicker layer of frosting to the cake, and smooth using a bench scraper. Then decorate as desired!

I decorated this cake as a cat for my husband's birthday, and if you want to create the same look please refer to the steps in my video :)