In a mixing bowl, use an electric mixer to blend the softened butter, all-purpose flour, powdered sugar, and fine salt together. Initially, the mixture may appear crumbly, but it will eventually form a soft dough as you continue mixing.

If the dough doesn't come together, you can microwave the bowl for 2–3 seconds at a time to slightly soften the butter, but be careful not to melt it.

Once the dough is formed, fold in 3 tablespoons of rainbow nonpareil sprinkles until they are evenly distributed throughout the mixture.

Transfer the dough onto a large piece of plastic wrap and shape it into a rectangle about ⅓ inch thick. Sprinkle the remaining 1 tablespoon of sprinkles on top and gently press them into the dough to ensure they stick.

Wrap the dough tightly in the plastic wrap and refrigerate for an hour, or until it becomes firm.

While the dough chills, preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper or Silpat mats for better cookie shape retention.

After chilling, unwrap the dough and lay the plastic wrap flat on your counter for easy cutting.

Using a sharp knife, cut the dough into small squares, approximately ½ inch in size. Remember that the cookies will spread a little while baking, so place them about 1 inch apart on the prepared baking sheets.

Bake one tray at a time for 10–12 minutes, or until the cookies are set and the bottoms are lightly golden. Rotate the pan halfway through baking for even results.

Once baked, transfer the pan to a wire rack and allow the cookies to cool completely on the pan.