Cream the butter and sugar together in a stand mixer with the paddle attachment until fluffy and light. Add the egg yolk and vanilla extract. Mix to combine.
In a separate bowl, mix together the flour and salt. Gradually add to mixer, scraping the bowl as needed. Dough will be dry and crumbly at first. Keep mixing until the dough comes together.
Line a sheet pan with parchment paper or a silicone baking mat. Use a small #50 scoop to portion cookie dough balls, about 1 Tablespoon each, and place onto the prepared cookie sheet. NOTE: Fit all cookies on the same tray for now, so the dough balls can chill all at once.
Roll balls with hands to smooth the outside completely. Then make a dent in the center of each dough ball with a ½ teaspoon or your thumb. TIP: Indent immediately after rolling the balls, while the cookie dough is still warm from your hands, to help prevent cracks. Dip the spoon, or your thumb, in granulated sugar if it sticks to the dough.
CHILL the indented dough balls for 20 minutes in the freezer until cold and firm to the touch. This helps prevent spreading too much.
While the dough is chilling, preheat the oven to 350 degrees Fahrenheit.
Place cookies 2 to 3 inches apart on a parchment or silicone lined baking sheet. (Do not bake the cookies all at once on the same tray they were frozen on. I fit 15 cookies on my half-sheet tray.) Leave remaining cookies in freezer until ready to bake.
Quickly fill cookie wells with a rounded ½ teaspoon of lemon curd. Bake in batches at 350˚F for 14 minutes or until cookies are set and edges are very lightly browned. Allow cookies to cool on the cookie sheet.
In a small bowl, stir together the powdered sugar and lemon juice to form a glaze. Drizzle glaze over the cookies OR place the glaze in a zip-top bag, make a small cut in the corner, and pipe a zig-zag over the cooled cookies. Allow the glaze to set before eating or storing.
Notes
You may use any jam, jelly, or preserves with or without seeds to fill these thumbprint cookies with. If using jam, place it in a small microwave-safe bowl. Nuke for 5-10 seconds, and stir until jam is smooth. The jam should not be hot, but be more fluid.
To Store: Store these lemon thumbprint cookies in an airtight container at room temperature. There’s no need to refrigerate them. These cookies will keep for at least a week before starting to dry out and get crumbly.
To Freeze: Allow cookies to cool for a few hours so the curd sets, then stack the cookies in an airtight container with parchment paper between the layers. The cookies will keep in the freezer for up to 1 month. NOTE: You may want to leave the glaze off if choosing to freeze the cookies.