Preheat the oven to 350 degrees F. Grease a sheet pan with nonstick spray. Line 3 (12-cup) cupcake or muffin pan with paper cupcake liners.
For the Peanut Brittle:
Combine the sugar, peanuts, and corn syrup in a saucepan. Cook on medium heat until the mixture is golden in color. Remove the saucepan from the heat. Stir in the butter and baking soda. Working quickly, spread the mixture onto the greased sheet pan. Allow the mixture to cool slightly before using a spatula to spread the mixture out even further. Once cooled a little more, stretch the candy out to be thin. Once fully cooled, approximately 30 minutes, break the candy into small pieces. You will not need all this brittle for the cupcakes. The extra can be stored in a plastic storage container for snacking or a second batch of cupcakes!
For the Buttermilk Pound Cake:
In a large bowl, beat together the sugar and butter until smooth. Add in the eggs, 1 at a time. Scrape down the bowl and mix after each egg addition. In a separate bowl, sift together the cake flour, all-purpose flour, baking powder, salt, and baking soda. In a third bowl, mix together the buttermilk and vanilla extract. Add the dry ingredients and buttermilk mixture to the butter-egg mixture, alternating between the dry and wet ingredients. Fold in the chopped peanut brittle.
Using a medium-size ice cream scoop, scoop the batter into the prepared cupcake liners Add a layer of mini chocolate shell candies. Bake until the tops of the cupcakes lightly spring back when pressed and a toothpick inserted in the center comes out clean, about 20 to 22 minutes.
Warm the butterscotch topping, and then add the cream and salt, mixing until smooth. Allow the mixture to cool, and then transfer to a disposable pastry bag fitted with a small tip and use to fill the cupcakes. This is a little sticky and messy!
In a food processor, blend the butter, sugar, cocoa, and salt until smooth. Add the corn syrup and vanilla extract. Blend again, making sure to scrape the sides of the food processor. Add the chocolate and pulse until completely combined. It should be smooth and creamy.
Place the brown sugar, salt, butter, corn syrup, and vanilla extract into a microwave-safe container, and cook on high power for 1 minute. Remove and stir until smooth. Place the container back in the microwave and cook again for another 2 minutes, or until the mixture is bubbling.
Remove the container from the microwave and stir in the baking soda. It will foam up. Spray a large stainless steel bowl with nonstick spray, and place the popcorn in it. Pour the hot mixture from the microwave over the popcorn, and toss until evenly coated. Spread the popcorn over a sheet tray and move it around until it is all separated. Allow the mixture to cool before layering on top of the fudge icing.
In a bowl, whisk together the egg whites and sugar. Heat them over a bain marie until hot and the sugar is melted. Pour the hot egg white-sugar mixture into the bowl of an electric mixer and whisk on high until the mixture is cooled. Switch to a paddle attachment. With the mixer running on medium-high speed, gradually add the softened butter until the frosting is thick, smooth and glossy. Add the vanilla extract, and mix until combined.
For the assembly:
Fill the cooled cupcakes with the butterscotch caramel. Spread the fudge icing on top. Sprinkle the caramel corn over the icing and top with a dollop of the vanilla buttercream. Top the buttercream with a mini chocolate shell candy.