Preheat the oven to 350°F. Prepare three 6-inch or two 8-inch cake pans by spraying the sides with a cooking spray (Baker's Joy is my favorite) 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 (a handheld mixer works fine too!), 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. It will look curdled at this point, but don’t worry – it will become smooth cake batter at the end.

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. The batter will be slightly thick.

Pour the batter evenly into the prepared cake pans and bake for 30-35 minutes. They're done when they spring back to the touch and a toothpick inserted into the middle comes out clean. 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. Make sure they're entirely room temperature before applying any frosting.

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. If you have a candy thermometer, you’ll want to cook this until it reaches 225F/107C. That is the temperature that will thicken the caramel while keeping it nice and soft at room temperature.

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. You can either leave the container on the counter for about 2 hours or speed the cooling process slightly by placing the container in the refrigerator. It will thicken significantly as it cools.

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 (to prevent ingredients flying out of the bowl), then turn the mixer to low and mix until all ingredients start to come together.

Remove the kitchen towel, scrape down the bowl and paddle, then continue mixing on low speed until uniform and smooth, 2-3 minutes.

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 (about ¾ of the height of the chocolate buttercream) with thick caramel filling. Here's a detailed tutorial on this filling method in case this is your first time working with a soft 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 of piping the chocolate buttercream circle and filling in the center with caramel and shortbread cookie crumbles, followed by a thin layer of chocolate buttercream. Place the final layer on top, upside down so that the bottom of the cake layer is the top of the cake. Crumb coat the cake with chocolate buttercream and place it in the refrigerator for at least 20 minutes to let everything set up.

To create the look pictured, frost the cake with chocolate buttercream and use a small icing spatula to create a textured finish. I held the icing spatula against the cake and moved it slightly up and down while rotating the turntable to mimic the texture of the chocolate on a Twix bar. Place the cake into the refrigerator for 10-20 minutes to let the frosting chill while you prepare the caramel drip.

To Create the Caramel Drip: Add about ½ Cup of thick caramel filling into a microwave safe bowl and microwave in 5-10 second increments until it's slightly above room temperature and ideal drip consistency. Pour onto the top of the chilled cake and then use an angled spatula to gently push the caramel over the edges and create a curtain drip. Place the cake back into the refrigerator for 5-10 minutes to let the drip set.

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