Preheat the oven to 325°F (165°C). Line two large baking sheets with parchment paper.

In a medium bowl, using a handheld electric mixer, beat together the butter and 2/3 cup of the granulated sugar until combined. The mixture may look slightly gritty, which is normal.

Add the flour and blend until fully incorporated. If using vanilla, blend it in after the flour.

Check the dough by pinching off a small piece and pressing it with your fingers. The dough should feel soft, buttery, and smooth. If it feels crumbly, continue kneading gently until it comes together easily.

Using a cookie scoop, portion the dough into 1-inch balls. Knead each portion briefly and roll between your palms until smooth.

Roll each dough ball in the remaining 3 tablespoons of granulated sugar until lightly coated. Place them on the prepared baking sheets, spacing about 2 inches apart.

Using the bottom of a measuring cup or glass, gently press down on each dough ball to flatten. Sprinkle with nonpareils if using, then lightly press again so they stick. The dough should be about 1/8 to 1/4 inch thick.

Bake for 14–16 minutes, until the cookies are just slightly golden around the edges and on the bottom.

Remove from the oven and let the cookies rest on the baking sheets for at least 10–15 minutes. Do not skip this step. Then eat or transfer to a cooling rack to cool completely.