In a large bowl, cream butter and sugar until light and fluffy. Beat in eggs, lemon zest and lemon juice. In another bowl, whisk flour, oats, baking powder and salt; gradually beat into creamed mixture. Refrigerate, covered, 2 hours or until firm enough to shape., Preheat oven to 375°. Shape level tablespoons of dough into balls; place 2 in. apart on parchment-lined baking sheets. Coat bottom of a glass with cooking spray, then dip in sugar. Press cookies with bottom of glass to flatten, redipping in sugar as needed., Bake until edges are light brown, 6-8 minutes. Remove from pans to wire racks to cool.