Preheat your oven to 325 degrees F. Spray three 8-inch pans with nonstick spray, line the bottom with parchment paper and spray again. Set aside.

In a medium size mixing bowl, whisk the cake flour, cocoa powder, and salt to combine. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugars on medium-high until light in color and fluffy in texture, about 3 minutes.

With the mixer on medium speed, add the eggs one at a time, making sure to scrape down the sides and bottom of the bowl between each addition. Continue to beat on medium-high speed for about 2 minutes, until smooth. Add the vanilla and stir to combine.

Turn the mixer to low speed and add 1/3 of the flour mixture to the butter mixture. Mix until just incorporated and then add half of the buttermilk, followed by another 1/3 of the flour mixture, the second half of the buttermilk and lastly, add the remaining flour mixture and mix until just incorporated.

In a small bowl, combine the vinegar and baking soda. The mixture will bubble. Add the mixture to the cake batter and stir on low until combined. Scrape down the sides of the bowl and mix again for another 20 seconds.

Gently fold in the sour cream.

Divide the batter evenly between the prepared pans (about 20 ounces in each pan) and bake for about 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it.

Remove the cake pans from the oven and let the layers cool in pans for 15 minutes before inverting onto cooling racks to cool completely.

In a bowl fitted with a paddle attachment, beat the butter on medium high until light in color and fluffy in texture, about 3 minutes.

In a microwave safe bowl, heat the cream for about a minute. Pour the hot cream over the chocolate chips. Stir to melt the chocolate and create a smooth ganache. Set aside to cool slightly.

With the mixer on medium speed, gradually add the chocolate ganache to the butter. Scrape the sides of the bowl and mix again until the mixture is smooth and silky.

With the mixer on low speed, gradually add the powdered sugar. Beat on low speed for 1 minute, or until the powdered sugar is fully incorporated.

Add the vanilla and salt and beat until combined.

Stream in 1/2 cup of the salted caramel.

Scrape down the sides and bottom of the bowl again, and then turn the mixer to medium-high speed and beat the buttercream for about 3 to 5 minutes. The mixture should be silky and smooth and lighter in color.

If the buttercream seems too soft, you can refrigerate it for about 15 to 20 minutes before using.

Place the first cake layer on a cake board and spread about 3/4 to 1 cup buttercream over the top.

Sprinkle about 1/2 cup of the shortbread crumbs and then some chopped Twix on top. Pat into place.

Drizzle a couple tablespoons of the caramel over the crumbs.

Place the second cake layer on top and repeat these steps.

Place the final cake layer, top side down, on the second layer of filling.

Apply a thin coat of frosting around the entire cake to lock in the crumbs. Freeze for about 10 to 15 minutes.

After the crumb coat is set, continue frosting and decorating the cake with remaining buttercream.