Step 1: Preheat oven to 325°F (160°C). Line 2 large baking sheets with parchment paper.
Step 2: In a medium bowl, using a handheld electric mixer, beat together the room temperature butter and 2/3 cup granulated sugar until combined. (It might be a little gritty – that's perfectly normal.)
Step 3: Add in the 2 cups + 1 tablespoon all-purpose flour and blend well. If using, mix in the 1 teaspoon vanilla extract until just combined.
Step 4: After blending the dough, pinch off a small piece and press it with your fingers. It should come together very easily and be soft, buttery, and smooth. If it feels crumbly at all, continue kneading it a little by hand until it's pliable and smooth.
Step 5: Using a 1-inch cookie scoop (or about 2 tablespoons), scoop out portions of dough. Knead and roll each portion in the palms of your hands until they feel soft and smooth and form uniform balls.
Step 6: Gently roll the dough balls in the remaining 3 tablespoons of granulated sugar until lightly coated. Transfer them to the prepared baking sheets, spacing them about 2 inches apart.
Step 7: With the bottom of a measuring cup or glass, gently press down on the balls to flatten them to about 1/8 to 1/4 inch thick. Sprinkle with nonpareils or rainbow sprinkles (if using) and lightly press down again just so they stick.
Step 8: Bake for 14-16 minutes or until just slightly golden around the edges and on the bottom. Do not overbake to maintain their soft texture.
Step 9: Remove from the oven and let rest on the baking sheets for at least 10-15 minutes (do not skip this crucial step, as it helps them set!). Then, enjoy warm or transfer to a cooling rack to cool completely.