Cream the butter with a hand mixer until fluffy and light, about 2 minutes.
Add in egg, vanilla, and optional zest and mix until combined. The mixture will look a bit curdled, but will smooth out once the dry ingredients are incorporated.
Add the powdered sugar and flour to the butter and slowly incorporate, starting your mixer on low. Once all of the dry ingredients are incorporated, continue to mix at a medium speed until the dough starts to form.
Roll generous tablespoonfuls of dough into a ball and coat with granulated sugar.
Dip the bottom of a round ½ teaspoon measuring spoon or your thumb into granulated sugar and create a thumbprint on each cookie, careful not to go too close to the bottom of the cookie.
Chill the cookie dough balls for 30 minutes, either on the parchment-lined sheets or a smaller pan to save refrigerator space. While the dough chills, preheat the oven to 375°F.
Place the cookie balls on the parchment-lined sheets, spacing them apart by 2 inches.
Add a spoonful of lemon curd to each cookie, trying not to overfill. A piping bag or a zip-topped bag makes this easier.
Bake, until the edges of the cookie are just beginning to brown, about 15 minutes. If you are baking two sheet pans of cookies at the same time, swap them from top rack to bottom (and vice versa) after about 8 minutes of cooking. Remove from pan after baking and cool on a wire rack.
Nutrition facts are sometimes provided below and are calculated using an online calculator. With specific brands of ingredients and additions, omissions, or substitutions the nutrition facts may change. We encourage you to use your own nutrition facts calculator to obtain the most accurate nutrition facts for your meal.