For the cake batter: Preheat the oven to 350 degrees F; line cupcake pans with 24 cupcake liners.
Put the butter in the bowl of a stand mixer with a paddle attachment and beat slowly until it is smooth and creamy. Add the granulated sugar and salt and cream at moderate speed until the mixture is light and fluffy, 8 to 10 minutes.
Add the eggs and egg whites together a little at a time, beating after each addition until the eggs are absorbed before adding more. Mix until light and fluffy, scraping down the sides of the bowl to insure even mixing, about 5 minutes.
Sift together the cake flour and baking powder in a medium bowl and set aside. Mix the milk and vanilla together in a separate bowl. Add one-quarter of the flour mixture to the butter and egg mixture and beat just until blended in. Add one-third of the milk mixture and beat just until blended in. Repeat with the remaining ingredients, alternating the flour with the milk, until all the ingredients are used. Scrape down the sides of the bowl occasionally for even mixing.
Scoop some batter into each liner and bake until an inserted toothpick comes out clean, 20 to 22 minutes. Let cool completely before frosting.
For the custard frosting: Using an electric mixer with a whip attachment, whip the cornstarch and egg yolks in a stainless steel bowl until light and fluffy. Warm the whipping cream and confectioners' sugar in a saucepan over medium heat. When the cream mixture reaches about 100 degrees F, add about half of it to the egg yolks, whisking, and then transfer the mixture back to the saucepan. Continue cooking the yolk and cream mixture until it reaches a boil (about 180 degrees F). Remove from the heat immediately and pour into a clean stainless steel bowl. Whip in the butter and vanilla until smooth and creamy. (The mixture may be stored in an air-tight container up to 5 days.)
For the brulee sugar mixture: Mix together the brown sugar and granulated sugar in a medium bowl.
Top each cupcake with 1 1/2 ounces of the custard frosting. Sprinkle on some of the brulee sugar mixture and torch. Repeat several times to desired thickness.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.