In a medium saucepan over medium heat, whisk together egg yolks, granulated sugar, and salt until the mixture becomes pale in color. Gradually incorporate the cornstarch followed by the whole milk, stirring continuously. Cook for 8-10 minutes until thickened, and the custard coats the back of a spoon. Remove from heat, stir in vanilla, lemon zest, and unsalted butter until smooth. Cool completely and refrigerate for 2 hours.

In a large mixing bowl, cream together granulated sugar and unsalted butter until the mixture is light and fluffy, about 3-4 minutes. Add the egg yolks one at a time, mixing well after each addition, along with the vanilla bean paste. In a separate bowl, combine flour, baking powder, baking soda, and salt, then gradually mix these dry ingredients into the wet mixture until a balanced dough forms.

Once the custard has chilled and thickened, gently fold half of the custard into the prepared cookie dough until fully integrated.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper. Using a cookie scoop or spoon, drop rounds of dough onto the lined baking sheet, spacing them about 2 inches apart. Bake for 10-14 minutes, or until the edges are lightly golden.

After baking, remove the cookies from the oven and let them cool on the baking sheet for about 5 minutes before transferring to a wire rack. Sprinkle a thin layer of granulated sugar over each cookie, using a kitchen torch to caramelize the sugar until golden and crisp.

Allow the cookies to cool for a few minutes more after brûléeing, letting the sugar crust harden. Consider garnishing with fresh thyme, a dusting of powdered sugar, or lemon peel. Enjoy!