Preheat the oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.

In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes. Ensure thorough creaming for optimal texture.

Add sour cream and vanilla extract, mixing until well combined.

Add egg whites in two batches, mixing well after each addition. Scrape down the sides of the bowl as needed for thorough incorporation.

Combine dry ingredients in a separate bowl, and mix milk and water in a small measuring cup.

Add half of the dry ingredients to the batter, mix until well combined. Add the milk mixture, mix well, and then add the remaining dry ingredients. Scrape down the sides of the bowl for thorough mixing.

Fill cupcake liners about halfway. Bake for 15-17 minutes, or until a toothpick inserted comes out with a few crumbs.

Allow cupcakes to cool for 2-3 minutes, then transfer to a cooling rack to finish cooling.

While cupcakes cool, prepare the pastry cream. Beat egg yolks in a medium-sized bowl and set aside.

In a saucepan, mix sugar, cornstarch, and milk until smooth. Cook over medium-high heat, stirring continuously, until the mixture thickens and bubbles.

Reduce heat to medium and simmer for 2 minutes, then remove from heat.

Gradually add a bit of the milk mixture to the beaten egg yolks, whisking together. Add the egg mixture back to the milk mixture to prevent cooking the yolks.

Place the pan back on heat and bring to a light boil, stirring continuously. Allow to boil for 2 minutes.

Remove from heat, add butter and vanilla extract, stir until smooth, and set aside to cool to room temperature.

For the frosting, beat butter and shortening together in a large mixer bowl until smooth.

Add half of the powdered sugar and mix until smooth. Add 3 tablespoons of caramel sauce and mix.

Add the remaining powdered sugar and mix until smooth. Adjust consistency with additional caramel sauce and water or milk.

Use a cupcake corer or knife to remove centers from cupcakes, then fill with pastry cream.

Pipe frosting onto cupcakes using a desired tip.

Dip cupcakes, one at a time, into sugar in a small bowl.

Refrigerate cupcakes until frosting is cool and firm (about 30 minutes to an hour).

Use a butane kitchen torch to caramelize the sugar on top.

Refrigerate cupcakes until ready to serve, best served cool but not cold.