Make Pastry Cream: Heat 2 cups of whole milk in a saucepan over medium heat until steaming. In a bowl, whisk together 4 egg yolks, ½ cup granulated sugar, zest of 1 lemon, salt, vanilla bean paste, and cornstarch. Gradually pour hot milk into the egg mixture, whisking constantly. Return to saucepan, cook over medium heat for 5-7 minutes until thick. Remove from heat, stir in butter, chill covered in refrigerator.

Prepare Dough: Whisk together 2 cups all-purpose flour, baking powder, baking soda, and salt in a bowl. Beat ½ cup softened unsalted butter and 1 cup granulated sugar until fluffy. Add 1 egg, vanilla extract, and another lemon zest, stirring well. Mix in dry ingredients until just combined.

Shape Cookies: Preheat oven to 350°F (175°C). Roll dough into 1-inch balls, coat in a mixture of ¼ cup granulated sugar and lemon zest. Flatten each ball slightly, place on lined baking sheet, leaving space between cookies.

Bake: Bake for 9-10 minutes until set and edges golden, but centers remain soft. Cool on baking sheet for 5 minutes before transferring to wire rack.

Assemble: Once cooled, pipe chilled lemon pastry cream onto each cookie. Sprinkle sugar evenly over pastry cream and use a kitchen torch to caramelize it. Move the torch to avoid burning spots.