For the topping:
For the cupcakes:
To make the pastry cream, combine 6 tablespoons of the milk with the cornstarch in a small bowl and whisk to blend well. Set aside. In a medium saucepan, combine the remaining 2½ cups plus 2 tablespoons of milk with the sugar, salt and vanilla bean. Warm over medium-high heat until the sugar is dissolved. Meanwhile, place the egg and egg yolks in a large liquid measuring cup and whisk lightly. Blend in the cornstarch mixture.
When the milk-sugar mixture just begins to simmer, slowly pour a portion of the warm milk into the measuring cup with the eggs, whisking constantly to prevent curdling. Continue until most of the milk has been mixed in. Return the mixture to the saucepan over medium-high heat. Bring to a boil, then cook until thickened, about 1 minute, stirring constantly and scraping the bottom of the pan to prevent burning. Remove the pan from the heat, whisk in the butter and vanilla until smooth. Transfer to a bowl or storage container, press a piece of plastic wrap directly on the surface of the pastry cream to prevent a skin from forming, and chill thoroughly.
To make the cupcakes, preheat the oven to 350˚ F. Line cupcake pans with paper liners. In a small bowl, combine the flour, baking powder and salt. Whisk to blend. In the bowl of an electric mixer, combine the butter and sugar and beat on medium-high speed until light and fluffy, about 2-3 minutes. Blend in the eggs one at a time. Blend in the vanilla. With the mixer on low speed, add in the dry ingredients in three additions alternately with the buttermilk, beginning and ending with the dry ingredients and mixing each addition just until incorporated.
Divide the batter between the prepared liners filling each about half to two thirds full. Bake the cupcakes, rotating the pans halfway through baking, until just set and a toothpick inserted in the center comes out clean, about 18 minutes. Transfer to a wire rack to let cool completely.
To assemble the cupcakes, use a small paring knife to cut out a wide cone from the top of the cupcake. Transfer the pastry cream to a pastry bag fitted with a plain round tip (I use Ateco #809). Fill the cones with a generous layer of the pastry cream. (If making in advance, the cupcakes can be covered and refrigerated at this point.) When ready to serve, sprinkle a layer of sugar over the surface of the pastry cream and caramelize to a golden brown with a kitchen torch. Serve immediately.