Preheat the oven to 350 degrees F. Line two standard muffin pans with 24 cupcake liners.
For the salted caramel: Combine the sugar, 1/3 cup water and the corn syrup in a small saucepan. Stir with a wooden spoon and bring to a simmer over medium heat. Simmer, swirling the pot occasionally, until the mixture is a beautiful amber color (watch closely). Once the desired color is reached, immediately add the butter and stir with a wooden spoon until completely incorporated. Remove from the heat. Add the heavy cream and stir to incorporate. Stir in the coarse salt. Pour 1/2 cup of the salted caramel into a small bowl or measuring cup, and the remainder into another bowl. Cool completely, stirring occasionally. (You can speed up the cooling by putting the caramel into the refrigerator, but it must be pourable when you are ready to assemble the cupcakes.)
For the brittle: Butter a small baking sheet and set aside. In a small but deep saucepan, combine the sugar, corn syrup, butter and 2 tablespoons water. Bring to a full boil over high heat, and boil until a candy thermometer reaches 280 degrees F. Remove from the heat and immediately stir in the baking soda, and then the pecans. Pour onto the prepared pan and spread with a rubber spatula. Top with the coconut. Allow to cool. Break the brittle into pieces of desired size.
For the cupcakes: In a medium bowl, sift together the all-purpose flour, cake flour, baking powder and kosher salt and set aside. Combine the milk and vanilla and set aside. In the bowl of an electric stand mixer with the paddle attachment, cream the butter and sugar at medium speed until light and fluffy, about 5 minutes, scraping down the sides of the bowl as necessary. Once incorporated, add the eggs, one at a time, at low speed. Be sure to incorporate after each addition, scraping down the sides of the bowl as necessary. Do not over beat. Once incorporated, add the flour mixture and the milk mixture. Alternate with three parts flour and two parts milk, being sure to scrape down the sides of the bowl as necessary (we don't want any butter chunks stuck to the bottom). Using a 1/4-cup ice cream scoop, fill each cupcake liner with batter.
Bake until a skewer inserted in the center comes out clean, 18 to 20 minutes. Transfer the pans to cooling racks and cool for about 5 minutes, and then transfer the cupcakes to the racks to cool completely before filling and frosting.
While the cupcakes are baking, make the buttercream: Combine the sugar and egg whites in the bowl of a stand mixer. Place the bowl over simmering water and whisk continuously until the sugar is melted. Remove from the heat, place under the mixer with a whisk attachment and whisk at medium-high speed until soft peaks form and it cools to room temperature. Add the butter and beat until incorporated. With the mixer running, slowly beat in the 1/2 cup reserved salted caramel until the mixture is homogenous. Set aside at cool room temperature until you are ready to frost the cupcakes.
To assemble: Use a small serrated knife, core each cupcake about 1 1/4 inches wide at the top and halfway into the cake. (Discard the cores-or nibble while you work.) Fill each with about 1 tablespoon of the remaining salted caramel. Sprinkle with about 1/4 teaspoon coarse salt. Frost each cupcake with a generous amount of caramel Swiss buttercream and garnish with crumbled pecan coconut brittle. Eat.