In a large bowl, cream butter and sugar until light and fluffy, 5-7 minutes.
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.
