Preheat your oven to 350°F (175°C) and line your baking sheets with parchment paper.
In a large mixing bowl, use an electric mixer to cream together the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.
Add the large egg yolk and vanilla extract to the butter-sugar mixture. Beat until fully incorporated.
In another bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined.
Using a cookie scoop, portion out the dough and roll each piece into a ball. If desired, roll them in sanding sugar for a sparkling finish.
Place the cookie dough balls on the prepared baking sheets, spacing them about 2 inches apart. Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Once baked, cool the cookies on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.