Preheat oven to 325°F (163°C). Line two large baking sheets with parchment paper.
In a medium bowl, use a handheld electric mixer to beat the butter and 2/3 cup sugar until combined. The mixture will be slightly gritty—this is normal.
Add in the flour and mix until well combined. If using vanilla, blend it in.
Roll the dough into 1-inch balls using a cookie scoop or your hands.
Roll each ball in the remaining 3 tablespoons of sugar to lightly coat, then place them on the baking sheets about 2 inches apart.
Flatten each dough ball slightly with the bottom of a measuring cup or glass. If using sprinkles, add them on top and press lightly so they stick. The dough should be about 3/16-inch thick.
Bake for 14-16 minutes, or until the edges and bottoms are just slightly golden.
Remove from the oven and let the cookies rest on the baking sheets for at least 10-15 minutes before transferring to a cooling rack.
Enjoy!