In a medium bowl, whisk together the flour and baking powder. Set aside.

In a large mixing bowl, cream together the butter, sugar, and vanilla extract until light and fluffy.

Add eggs one at a time, mixing well after each addition until fully incorporated.

Gradually mix in the flour mixture until combined. Knead the dough gently until it becomes firm and no longer sticky. Avoid adding too much flour, as the dough will firm up after chilling.

Wrap the dough in plastic wrap and refrigerate for at least 1 hour.

Preheat the oven to 375°F (190°C) and line baking sheets with parchment paper.

For round cookies, roll the chilled dough into 1-inch balls and place them 2 inches apart on the prepared baking sheets.

For swirl cookies, roll a small piece of dough on a lightly floured surface into a 6-inch rope (about ⅓ to ½ inch thick). Pinch one end and gently twirl the rest of the rope around to form a circular shape. Tuck the end underneath the cookie.

Arrange the shaped cookies on the baking sheets.

Bake for 8–10 minutes, or until the bottoms are lightly golden while the tops remain pale.

Remove from the oven and transfer cookies to a wire rack to cool completely before glazing.

In a bowl, whisk together the powdered sugar, vanilla extract, and milk until smooth and runny but thick enough to coat the cookies. Adjust with additional milk or powdered sugar as needed.

Dip the tops of the cooled cookies into the glaze, allowing the excess to drip back into the bowl.

Place the glazed cookies back on the wire rack and immediately sprinkle with nonpareil sprinkles while the glaze is still wet.

Let the glaze dry completely for 30–60 minutes before serving or storing.