Set oven to 350°F (175°C). Prepare 2–3 baking sheets by lining them with parchment paper; depending on your oven's capacity, you might need to bake in multiple batches.

Using a stand mixer with the paddle attachment, beat the butter and sugar on high speed until the mixture becomes light and fluffy. Incorporate the egg and vanilla, mixing until combined. In a separate small bowl, whisk together the flour, baking soda, and salt. Add this dry blend to the mixer, then add the sprinkles, and mix on low speed until just incorporated.

With a 2 Tbsp cookie scoop, drop portions of dough onto the lined baking sheets. Begin with 6 per sheet to verify they won't touch, then increase to 8 per sheet once you've determined their spread.

Shape each dough portion into a ball by rolling between your palms, then coat in the sugar before setting on the baking sheet. Keep any dough you aren't baking right away in the bowl and scoop just before baking.

Bake the cookies for 11–12 minutes, until they are puffy and the edges are set. Take them out of the oven and let them cool on the pan for 10 minutes; they will flatten as they cool. Move to a wire rack and let them cool completely. Continue the baking process with the remaining dough.