Preheat your oven to 350°F (175°C) and line a muffin pan with paper liners.

Whisk together granulated sugar, cornstarch, and flour in a saucepan, then gradually pour in heavy cream and milk, whisking continuously. Add egg yolks and vanilla, cook until thickened.

Sift together flour, cornstarch, baking powder, and salt. In a separate bowl, cream butter and sugar, then add eggs one at a time, alternating with the dry ingredients and milk.

Spoon the batter into muffin cups and bake for 18-20 minutes until a toothpick comes out clean. Allow to cool in the pan for 5 minutes before transferring to a wire rack.

Cut a cone out of the center of each cooled cupcake and fill with chilled pastry cream. Top with a dollop of pastry cream.

Sprinkle coarse sugar over the tops of each cupcake and caramelize using a kitchen torch until a crackly crust forms.