Begin by heating the whole milk in a medium saucepan over medium heat until it just begins to steam. In a separate bowl, whisk the egg yolks, granulated sugar, salt, vanilla bean paste, and cornstarch until smooth. Gradually pour the hot milk into the egg mixture while whisking continuously, then return to the saucepan and cook over low heat, stirring constantly, until thickened and bubbling. Remove from heat and refrigerate until chilled.

Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt; set aside. In another bowl, cream the unsalted butter with granulated sugar until light and fluffy. Add in the egg and vanilla extract, mixing until combined. Gradually incorporate the dry ingredients into the wet mixture until just combined, forming a soft cookie dough.

Using a spoon or cookie scoop, portion out about 2 tablespoons of dough for each cookie. Roll the dough into balls and then roll each ball in granulated sugar. Place them on a baking sheet lined with parchment paper, leaving space between them, and slightly flatten each ball with your palm.

Place the baking sheet in the preheated oven and bake the cookies for about 9-10 minutes, or until the edges are golden brown and the centers look set but still soft. After baking, let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.

Once cooled, fill a piping bag with your chilled pastry cream and pipe a swirl onto the center of each cookie. Sprinkle a thin layer of sugar on top of the cream and caramelize the sugar using a kitchen torch until crispy.