Step 1: Prepare the Lemon Curd (if making homemade): In a small saucepan, whisk together 1/2 cup granulated sugar, 2 large egg yolks, and 1/4 cup fresh lemon juice. Cook over medium-low heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 5-7 minutes). Do not boil. Remove from heat, stir in 2 tablespoons butter and 1 teaspoon lemon zest until butter is melted and incorporated. Transfer to a small bowl, cover with plastic wrap directly on the surface to prevent a skin from forming, and chill in the refrigerator until firm, at least 1 hour.
Step 2: Make the Cookie Dough: In a large bowl, cream together the softened butter and 1/2 cup granulated sugar until light and fluffy. Beat in the egg yolk and vanilla extract until well combined.
Step 3: Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour and salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed or by hand until just combined and a soft dough forms. Be careful not to overmix.
Step 4: Chill the Dough: Form the dough into a disc, wrap it tightly in plastic wrap, and chill in the refrigerator for at least 1 hour (or up to 3 days). This helps prevent spreading.
Step 5: Preheat Oven & Prepare Baking Sheets: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Step 6: Form the Cookies: Roll the chilled dough into 1-inch balls. Place them about 2 inches apart on the prepared baking sheets. Using your thumb or the back of a small measuring spoon, gently press an indentation into the center of each dough ball.
Step 7: Fill and Bake: Spoon about 1/2 to 1 teaspoon of the chilled lemon curd into each indentation. Bake for 12-15 minutes, or until the edges are lightly golden and the centers are just set. Do not let them get too brown.
Step 8: Cool: Let the cookies cool on the baking sheets for 5 minutes before carefully transferring them to a wire rack to cool completely. The lemon curd will set further as it cools.