Combine sugar and lavender buds in food processor and process until well combined. Reserve 1/2 cup sugar-lavender mixture. In mixer bowl, beat butter, 2 cups sugar-lavender mixture, egg, vanilla, lemon zest and juice until fluffy. Combine flour, baking powder and salt; stir into butter-sugar mixture just until blended. Refrigerate, covered, at least one hour. (Dough may be refrigerated, covered, overnight.) Roll part of dough 1/2-inch thick on lightly floured surface; keep remainder of dough refrigerated. Cut with 4-inch diameter cookie cutters; place on ungreased baking sheets. Sprinkle with reserved sugar-lavender mixture. Bake in preheated 375 F oven until set, but not brown (6-8 minutes). Allow to set on baking sheet 1 minute before removing to wire racks to cool.