Step 1: Prepare the Cookie Dough: In a large bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.
Step 2: In a separate bowl, whisk together flour, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix. Wrap the dough in plastic wrap and chill for at least 30 minutes.
Step 3: Make the Lemon Curd: In a small saucepan, whisk together egg yolks, granulated sugar, lemon juice, and lemon zest. Cook over low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 5-7 minutes). Do not boil.
Step 4: Remove from heat and stir in the butter until melted and smooth. Transfer the curd to a small bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and chill completely in the refrigerator (at least 1 hour).
Step 5: Prepare the Crème Brûlée Topping: In a small saucepan, whisk together heavy cream, egg yolk, granulated sugar, vanilla extract, and salt. Cook over low heat, stirring constantly, until slightly thickened (about 3-5 minutes). Do not boil. Remove from heat, transfer to a small bowl, cover, and chill completely.
Step 6: Bake the Cookies: Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop rounded tablespoons of chilled cookie dough and roll into balls. Place them 2 inches apart on the prepared baking sheets.
Step 7: Using your thumb or the back of a small spoon, create a shallow indentation in the center of each cookie dough ball. Bake for 10-12 minutes, or until the edges are lightly golden. Remove from oven and gently deepen the indentations if they've risen too much. Let cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Step 8: Assemble the Cookies: Once cookies are completely cool, spoon about 1/2-1 teaspoon of chilled lemon curd into each indentation. Then, top the lemon curd with about 1 teaspoon of the chilled crème brûlée topping, spreading it gently to cover the curd.
Step 9: Place the assembled cookies on a baking sheet and chill in the refrigerator for at least 30 minutes, or until the topping is firm.
Step 10: Torch the Topping: When ready to serve, remove cookies from the refrigerator. Sprinkle a thin, even layer of granulated sugar over the crème brûlée topping on each cookie. Using a kitchen torch, carefully caramelize the sugar until golden brown and bubbly. Serve immediately for the best crispy crust experience.