Preheat the oven to 350 degrees F. With an electric mixer, cream the butter, sugar, vanilla, and lemon zest together until smooth. Scrape the bowl with a spatula. Turn the mixer on low and add the salt. Add the flour a little at a time, until it is just combined.
Line your cookie sheets with parchment paper. Roll the dough into 1-ounce balls, about 1 1/2 tablespoons. Place them on the cookie sheets and press with your thumb to create a cavity in the center. It helps them to keep their shape if you hold the sides of the cookie with the other index finger and thumb. Press any cracks back into place.
Fill each indentation with a scant 1/4 teaspoon of lemon curd. (Use just a little more if filling with jam.) Bake for 20-22 minutes. Remove them from the oven and immediately sprinkle the edges with turbinado sugar.