In a medium saucepan, heat 2 cups of whole milk over medium heat until it starts to steam but doesn’t boil. Meanwhile, whisk together 4 egg yolks, ½ cup granulated sugar, the zest of 1 lemon, a pinch of salt, 1 teaspoon vanilla bean paste, and 2 tablespoons cornstarch in a bowl until pale. Gradually add the hot milk to the mixture, whisking constantly, then return it to the saucepan; cook on medium-low heat until thickened, about 5-7 minutes, stirring frequently. Once thickened, mix in 2 tablespoons of unsalted butter until fully combined, then chill in the refrigerator.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon baking powder, and ½ teaspoon baking soda with a pinch of salt until well combined. Set this dry mixture aside.

In a large mixing bowl, using an electric mixer, cream together ½ cup of softened unsalted butter and 1 cup of granulated sugar until light and fluffy, about 3-5 minutes. Add in 1 room temperature egg and the zest of one lemon, mixing on medium speed until the batter appears pale and smooth.

Gradually incorporate the dry flour mixture into your butter mixture, stirring until just combined. Use a cookie scoop to portion out the dough and roll each scoop into a ball. Then, generously coat each ball in lemon sugar.

Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper. Place the dough balls on the sheets, slightly flattening them with your palm. Bake for 9-10 minutes, or until the edges are lightly golden and the centers look slightly undercooked. Allow the cookies to cool for 5 minutes before transferring them to a wire rack.

Once your cookies are completely cool, take the chilled pastry cream from the refrigerator. Using a piping bag, pipe a generous amount of lemon pastry cream onto half of the cookies. Sprinkle sugar evenly over the cream layer.

Using a kitchen torch, gently caramelize the sugar on top of the pastry cream until it is golden brown and bubbly.