Step 1: In a medium bowl, whisk together the all-purpose flour, baking powder, and kosher salt. Set aside.
Step 2: In a large bowl, using an electric mixer, cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.
Step 3: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 4: Gradually add the dry flour mixture to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 5: Divide the dough in half. On a lightly floured surface or parchment paper, shape each half into a log about 1 1/2 to 2 inches in diameter. If using sprinkles, spread them out on a plate and roll the dough logs in the sprinkles to coat evenly.
Step 6: Wrap each dough log tightly in plastic wrap. Refrigerate for at least 1-2 hours, or until firm. Chilling is crucial for the cookies to hold their shape during baking.
Step 7: Preheat your oven to 375°F (190°C). Line two baking sheets with parchment paper.
Step 8: Unwrap the chilled dough logs. Using a sharp knife, slice each log into 1/4-inch thick rounds. Place the cookie rounds about 1 inch apart on the prepared baking sheets.
Step 9: Bake for 10-12 minutes, or until the edges are lightly golden. The centers should still look pale. Do not overbake.
Step 10: Remove from the oven and let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.