Prepare the Pastry Cream: In a medium saucepan, heat 2 cups of whole milk over medium-low heat until steaming but not boiling. Whisk together 4 egg yolks, 1 cup of granulated sugar, 2 tablespoons of lemon zest, ¼ teaspoon of salt, 1 tablespoon of vanilla bean paste, and 2 tablespoons of cornstarch in a bowl until pale and creamy.

Combine Mixtures: Gradually pour the hot milk into the egg yolk mixture while whisking continuously. Return the combined mixture to the saucepan over medium heat, whisking constantly for 5–10 minutes until thickened.

Chill the Pastry Cream: Remove from heat and stir in 2 tablespoons of unsalted butter until melted. Transfer to a bowl, cover with plastic wrap, and refrigerate for at least 2 hours.

Prepare the Cookie Dough: Preheat your oven to 350°F (175°C). In a large mixing bowl, combine 2 cups of all-purpose flour, 1 teaspoon of baking powder, ½ teaspoon of baking soda, and ¼ teaspoon of salt.

Cream Butter and Sugar: In another bowl, beat together ½ cup of softened unsalted butter and 1 cup of granulated sugar until light and fluffy. Add 1 egg, 1 teaspoon of vanilla bean paste, and 2 tablespoons of lemon zest.

Combine Dry and Wet Ingredients: Gradually add dry ingredients to the wet mixture until a soft dough forms.

Shape the Cookies: Portion out dough and roll into balls. Roll in a mixture of ½ cup granulated sugar and 2 tablespoons lemon zest if desired.

Bake the Cookies: Bake for 9–10 minutes until edges are golden brown but centers are soft. Let sit for 5 minutes before transferring to a wire rack.

Assemble the Cookies: Once cooled, pipe or spoon a dollop of lemon pastry cream on each cookie and sprinkle with granulated sugar.

Brûlée the Sugar Topping: Carefully caramelize the sugar with a kitchen torch until golden and bubbly, then cool slightly before serving.