Cream butter and sugars until light and fluffy, 5-7 minutes. Beat in lemon zest, juice and vanilla. In another bowl, whisk flour, poppy seeds and salt; gradually beat into creamed mixture. Shape into a disk; cover tightly. Refrigerate 4 hours or until firm enough to roll., Preheat oven to 350°. Roll dough between 2 sheets of waxed paper to 1/4-in. thickness. Cut with a floured 1-1/2-in. cookie cutter; reroll scraps. Place 1 in. apart on parchment-lined baking sheets. Bake until edges begin to brown, 10-12 minutes. Remove from pans to wire racks to cool.