These shortbread cookies travel well. "They don't contain chocolate, so they hold up in the heat," notes Laurie Lufkin. Don't need a big batch right now? Freeze the rolled dough and slice off a few rounds for quick-baking cookies in small amounts.
Sift together flour, baking powder, and salt; set aside. In a large bowl, cream butter and sugar together until light and fluffy. Mix in egg, vanilla, lemon zest, and lemon thyme until creamy. With your mixer running, slowly add flour mixture, blending well.
Turn out half the dough onto a sheet of wax paper and roll into a log about 2 inches in diameter. Roll dough in paper, twist the ends (it'll look like a huge piece of saltwater taffy), and refrigerate at least 1 hour. Repeat with remaining dough.
Heat oven to 350°. Line two baking sheets with parchment paper and set aside. When you're ready to bake, remove dough logs from refrigerator and let sit at room temperature 5 minutes. Unwrap each log. Sprinkle a few tablespoons large-grain sugar onto the waxed paper and spread to cover. Roll logs in sugar to coat all sides and cut into quarter-inch slices.
Bake 10-12 minutes until edges are lightly browned. Remove from oven. Let cookies rest on baking sheets; then let cool completely on racks.