Preheat the oven to 350°F (176°C) and line a cupcake pan with cupcake liners.
In a large mixing bowl, cream together butter and sugar until light and fluffy (about 3-4 minutes).
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.
In a separate bowl, combine the dry ingredients (flour, baking powder, and salt). In a small measuring cup, combine the milk and water.
Add half of the dry ingredients to the batter, then the milk mixture, and finish with the remaining dry ingredients, mixing well after each addition.
Fill cupcake liners halfway and bake for 15-17 minutes, or until a toothpick comes out clean.
Remove cupcakes from the oven and cool for 2-3 minutes, then transfer to a cooling rack.
Make the pastry cream: In a bowl, beat the egg yolks gently. Set aside. In a saucepan, mix sugar, cornstarch, and milk, stirring continuously over medium-high heat until thickened.
Temper the egg yolks by adding a little hot milk mixture, then combine the yolks with the rest of the milk mixture. Cook over medium heat for 2 minutes. Stir in butter and vanilla extract, then cool to room temperature.
Make the frosting: Beat butter and shortening until smooth. Gradually add powdered sugar and caramel sauce, mixing until smooth. Adjust the consistency with water or milk if needed.
Assemble the cupcakes: Use a cupcake corer or knife to remove the center of each cupcake. Fill with pastry cream.
Pipe frosting onto the cupcakes using a round tip (e.g., Ateco 808), creating a dome shape.
Dip the frosted cupcakes into sugar, covering the tops.
Caramelize the sugar: Refrigerate the cupcakes until the frosting is firm, then use a kitchen torch to caramelize the sugar on top.
Keep cupcakes refrigerated until ready to serve. They’re best served cool, but not cold.