Use your hands to rub the lemon zest into the granulated sugar until well combined and fragrant.
Add the butter and vanilla extract. Cream everything together until light and fluffy with an electric mixer.
Slowly add the flour, mixing until the dough resembles wet sand.
Form the dough into golf ball sized dough balls and spread them evenly across the surface of a nonstick cookie sheet, about 1 ½ inches apart.
Use your thumb to make a good indent into the center of each cookie.
Fill each indent with a teaspoon of lemon curd or a little more if it fits.
Bake the cookies in the oven at 350°F for 15 minutes.
Cool 5 minutes, then remove cookies to wire cooling racks and let the thumbprint cookies cool completely before serving.
Notes
If your cookie dough is too crumbly, slowly add 1 teaspoon of water at a time, mixing between until the dough holds together. These cookies will not brown in the oven. You’ll know they’re finished baking when they become matte rather than shiny.