In a medium saucepan, warm the whole milk over medium heat until it begins to steam, but do not let it boil. In a separate bowl, whisk together the egg yolks, granulated sugar, lemon zest, vanilla bean paste, and a pinch of salt until slightly thickened. Slowly temper the egg mixture by pouring in the warm milk while whisking continuously. Return to the saucepan and cook over medium heat, whisking constantly until thickened (about 5-7 minutes). Remove from heat, stir in unsalted butter until melted, and chill in the fridge.

Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper. In a bowl, whisk together flour, baking powder, baking soda, and a pinch of salt. In another bowl, cream softened butter and granulated sugar until fluffy. Add egg, vanilla, and remaining lemon zest, mixing well. Gradually add dry ingredients until a soft dough forms.

Scoop tablespoon-sized balls of cookie dough and roll in a mixture of granulated sugar and lemon zest. Place on baking sheets spaced 2 inches apart and flatten slightly. Bake for 9-10 minutes until edges are golden and centers are soft.

Let the cookies cool for 5 minutes on baking sheets before transferring to a wire rack to cool completely. Pipe chilled pastry cream on cookie bases, sprinkle with sugar, and use a kitchen torch to caramelize sugar until golden and crackly.