1) Stir flour with baking powder and salt in a medium bowl. Set aside.
2) Beat butter with sugar in a large bowl, using an electric mixer on medium-high, until fluffy. Beat in egg and vanilla. Gradually beat in flour mixture, just until combined. Gather into a ball. Divide dough in half, then roll each portion into a 2-in.-wide log. Wrap logs in wax paper and twist ends. Chill until firm, 1 to 1 1/2 hours.
3) Position racks in top and bottom thirds of oven. Preheat to 350F. Line 2 baking sheets with parchment.
4) Slice cookies into 1/4-inch rounds and arrange 1 inch apart on prepared sheets. Bake in top and bottom thirds of oven, switching halfway through, until cookies are golden, 10 to 12 min. Let cookies cool on sheets for 1 min, then transfer to a wire rack to cool completely. Repeat with remaining dough. Cookies will keep well, stored in an airtight container at room temperature, up to 5 days.
Make-ahead tip: Keep dough in freezer up to 1 month. Thaw in refrigerator overnight before using.
Makes 55 - 60 cookies
Candy Cane: Omit vanilla. Add 1/2 cup finely chopped candy canes to flour mixture.
Chocolate Orange: Omit vanilla. Add 1/2 cup mini chocolate chips and 2 tbsp. grated orange zest to flour mixture.
Mocha: Add 1/3 cup cocoa powder and 1 tbsp. instant-coffee granules to flour mixture.
Maple Walnut: Omit vanilla. Add 1/2 cup finely chopped toasted walnuts to flour mixture. Add 1/4 cup maple syrup with egg.
Mint Chocolate Chip: Omit vanilla. Add 1/2 cup mini chocolate chips to flour mixture. Add 1/2 tsp peppermint extract and 3-5 drops green food colouring with egg.
Candy Bar: Add 2 tbsp. each chopped PB chips, butterscotch chips, Skor bits and mini chocolate chips to flour mixture.
Birthday Cake: Add 1/2 cup sprinkles to flour mixture. Bake and cool cookies. Whisk 1/2 cup icing sugar with 1 tbsp. milk until smooth. Brush cookies with a thin layer of icing.
London Fog: Add 2 tbsp. finely ground Earl Grey tea leaves (about 8 tea bags) and 1/4 tsp orange zest to flour mixture. Bake and cool cookies. Whisk 1/2 cup icing sugar with 1 tbsp. milk until smooth. Brush cookies with thin layer of icing.