1. In a medium bowl, whisk flour, baking powder and salt.
2. In a large bowl, beat butter, sugar, thyme and lemon zest with an electric mixer on medium-high speed until light and fluffy, 2 to 3 minutes. Scrape down sides of bowl, then add yolk and beat until smooth. Stir in flour mixture until just incorporated.
3. Turn dough out onto a piece of waxed paper and shape it into a log about 8 inches long and 1 1/2 inches in diameter. Wrap in plastic and refrigerate for 2 to 24 hours.
4. Preheat oven to 350ºF. Slice dough into 1/3-inch-thick rounds; place at least 2 inches apart on ungreased baking sheets. Bake until pale golden around edges but still soft on top, 13 to 15 minutes. Let stand on baking sheets on wire racks for 5 minutes, then remove them with a metal spatula to wire racks to cool completely.