1. Butter an 8 1/2-inch cake or springform pan; set aside. Sift together flour and salt into a small bowl; set aside. Place butter in bowl of electric mixer fitted with paddle attachment. Cream until fluffy, 3 to 5 minutes. Add sugar, and continue to beat until very light in color and fluffy, occasionally scraping down the sides of the bowl with a spatula, about 2 minutes more. Add vanilla, if using. Add flour mixture, and combine on low speed, scraping with spatula if necessary, until flour is just incorporated and dough sticks together when squeezed with fingers.

2. Pat dough into prepared pan. If rolling out dough to cut into shapes, form into a flat disk; wrap in plastic. Chill until firm, at least 1 hour.

3. Roll into 1-inch balls with your hands; chill. Heat oven to 325 degrees. Bake until firm and just starting to color, 20 to 25 minutes. Allow to cool completely on a wire rack. Dip cooled shortbread balls into lemon-poppyseed glaze, allowing excess to drip off.