Preheat oven to 325 degrees F. Line 2 large baking sheets with parchment paper.
In a medium bowl, using a handheld electric mixer, beat together the butter and ⅔ cup sugar until combined. (It might be a little gritty - that's ok.)
Add in flour and blend well (then blend in the vanilla, if using.)
After blending the dough, pinch off a bit and press it with your fingers. It should come together very easily and be soft, buttery, and smooth. If it feels crumbly at all, keep kneading it a little.
Using a cookie scoop, scoop out 1-inch size balls, then knead and roll in the palms of your hands until they feel soft and smooth.
Gently roll the balls in the remaining 3 tablespoons of sugar until lightly coated; transfer to the baking sheets 2 inches apart.
With the bottom of a measuring cup or glass, press down on the balls to flatten. Sprinkle with some nonpareils (if using) and lightly press down again just so they stick.
Bake for 14-16 minutes or until just slightly golden around the edges and on the bottom.
Remove from the oven and let rest on the baking sheets for at least 10-15 minutes (don't skip this step!) Then eat or transfer to a cooling rack to cool completely. Enjoy!
