Preheat your oven to 325°F (165°C).

Line two large baking sheets with parchment paper to prevent sticking.

In a medium mixing bowl, beat the softened butter and 2/3 cup of sugar with a handheld electric mixer until combined. It may look a bit gritty, but that’s fine.

Add the flour to the butter mixture and blend until a dough begins to form.

If using, blend in the vanilla extract for a hint of extra flavor.

Pinch a bit of dough and press it between your fingers. It should come together easily and feel smooth and buttery. If it seems crumbly, continue kneading gently until it holds together.

Using a cookie scoop or a tablespoon, scoop out 1-inch sized dough balls.

Roll each ball between your palms until soft and smooth.

Gently roll each ball in the remaining 3 tablespoons of sugar to coat the outside.

Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.

Use the bottom of a glass or measuring cup to press down gently on each dough ball to flatten it.

If desired, sprinkle with rainbow nonpareils or other sprinkles, then lightly press down again so they stick to the dough.

The dough should be between 1/8 to 1/4 inch thick (roughly 3/16 inch).

Bake in the preheated oven for 14 to 16 minutes, or until the edges and bottoms are just slightly golden.

Remove from the oven and allow the cookies to rest on the baking sheet for at least 10–15 minutes before moving to a cooling rack. This helps them set and prevents breakage.