Prepare the Pastry Cream: Heat 2 cups of whole milk over medium heat until steaming. Whisk together 4 egg yolks, 1/2 cup granulated sugar, a pinch of salt, 1 tablespoon of vanilla bean paste, and 2 tablespoons of cornstarch until pale and thick. Gradually add hot milk and whisk. Return to heat until thickened, about 5-7 minutes. Stir in 2 tablespoons unsalted butter, cover, and chill for at least 30 minutes.

Make the Cookie Dough: Preheat oven to 350°F (175°C). Combine 2 cups all-purpose flour, 1 teaspoon baking powder, and a pinch of salt in a bowl. Cream 1/2 cup unsalted butter and 3/4 cup granulated sugar until fluffy. Mix in 1 egg and 1 tablespoon vanilla bean paste. Gradually combine dry and wet ingredients.

Shape the Cookies: Scoop cookie dough into balls and roll in granulated sugar. Place on parchment-lined baking sheets, flatten slightly.

Bake the Cookies: Bake for 9-10 minutes until edges are golden but centers are soft. Cool on sheets for 5 minutes, then transfer to a wire rack.

Assemble the Cookies: Pipe or spread chilled pastry cream on cooled cookies.

Caramelize the Sugar Topping: Sprinkle a thin layer of sugar on pastry cream and caramelize with a kitchen torch until golden brown. Allow to cool before serving.