Set oven to 325°F (165°C). Prepare 2 large baking sheets by lining with parchment paper.
In a medium bowl, using a handheld electric mixer, beat the butter with 2/3 cup (133 g) sugar until mixed. It may be slightly gritty - that's fine.
Incorporate the flour and mix thoroughly, then mix in the vanilla, if using.
Once the dough is mixed, take a small piece and squeeze it with your fingers. It should hold together easily and be soft, buttery, and smooth. If it seems crumbly, continue kneading a bit more.
With a cookie scoop, form 1-inch balls, then knead and roll them between your palms until they are soft and smooth.
Roll the balls gently in the remaining 3 tbsp (27 g) sugar to coat lightly; place on the baking sheets 2 inches apart.
Flatten the balls by pressing with the bottom of a measuring cup or glass. If using, add nonpareils and press lightly again to adhere. The dough should be between 1/8-1/4 inch thick, specifically 3/16 inch. They will appear small in circumference, but will spread to a normal cookie size.
Bake for 14-16 minutes or until just slightly golden on the edges and bottom.
Take out of the oven and let sit on the baking sheets for at least 10-15 minutes (do not skip this step!). Then eat or move to a cooling rack to cool completely. Enjoy!