Cream the softened unsalted butter and granulated sugar together in a mixing bowl until light and fluffy, about 2-3 minutes.
Add the large egg yolk and vanilla extract to the creamed mixture, mixing until well combined.
In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually mix this into the wet ingredients until just combined.
Chill the dough in the refrigerator for about 30 minutes, covered with plastic wrap.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Shape the chilled dough into balls and roll them in sanding sugar if desired. Place on the baking sheet, spacing them 2 inches apart.
Bake for 10-12 minutes, until the edges are lightly golden. The centers may look underbaked but will firm up as they cool.
Allow cookies to cool on the baking sheet for 5 minutes before transferring them to a cooling rack.
