Preheat the oven to 350°F. Prepare three 6-inch or two 8-inch cake pans by spraying the sides with a cooking spray and fitting a parchment paper circle to the bottom of the pan.

Sift the cake flour and then measure by spooning and leveling it in your measuring cup. Add the cake flour, baking powder, baking soda, and salt into a bowl and whisk to combine. Set aside.

In the bowl of your stand mixer, beat the butter on high for two minutes until it's light and creamy. Add in the sugar and continue to mix on medium-high for another two minutes. Turn the mixer to low and add the eggs and egg whites, one at a time, mixing until just combined and scraping down the bowl and paddle as needed. Add the vanilla and sour cream, turn the mixer to high, and beat for one full minute.

Add in the dry ingredients all at once and turn the mixer to low speed. Mix until just combined, then slowly pour in the milk. Continue to mix on low speed until just combined, about 30 seconds. Scrape down the sides and bottom of the bowl and give it a few stirs with a whisk to make sure there are no lumps.

Pour the batter evenly into the prepared cake pans and bake for 30-35 minutes. Let the cakes cool in the pan for five minutes before turning them out onto a wire rack for an additional few hours of cooling.

In a large saucepan over medium heat, add the sweetened condensed milk, butter, brown sugar, corn syrup, heavy whipping cream, and salt. Stir with a rubber spatula constantly until the mixture is fully combined and begins to bubble, about 5 minutes. Continue stirring until the caramel begins to thicken, 6-7 minutes.

Remove the caramel from the heat and mix in the vanilla extract. Pour into a separate container and let the caramel cool completely to room temperature before using it to fill the cake.

In a stand mixer fitted with a paddle attachment, whip the butter on medium-high speed until creamy and light in color, 4-5 minutes. Scrape down the bowl and paddle, then add the powdered sugar, cocoa powder, milk, salt, and vanilla extract. Cover the mixer with a clean kitchen towel, then turn the mixer to low and mix until all ingredients start to come together.

Once the vanilla cake layers have cooled completely, level and torte each layer to your desired height. Add about 1.5 Cups of chocolate buttercream into a piping bag and snip off about ½ inch opening.

Place the first layer on a turntable and pipe a circle of chocolate buttercream around the edge of the cake layer. Fill in the center with thick caramel filling. Fill the rest of the space with shortbread cookie crumbles, then pipe a little chocolate buttercream on top and spread it in a thin layer over the cookie crumbles.

Place the next cake layer on top and repeat the process. Place the final layer on top, upside down. Crumb coat the cake with chocolate buttercream and place it in the refrigerator for at least 20 minutes.

Frost the cake with chocolate buttercream and use a small icing spatula to create a textured finish. Place the cake into the refrigerator for 10-20 minutes.

To Create the Caramel Drip: Add about ½ Cup of thick caramel filling into a microwave safe bowl and microwave until it's slightly above room temperature. Pour onto the top of the chilled cake and create a curtain drip.

To finish, pipe a swirl border on top of the cake with Wilton Tip 4B. Top with shortbread cookie crumbs and chopped Twix bars.