Preheat oven to 350°F. Line two large baking sheets with parchment paper and pour the sprinkles in a small bowl. Set aside.
In a large mixing bowl, add butter and sugar. Beat with a hand mixer or a stand mixer fitted with a paddle attachment until creamed together, about 1 minute.
Add egg and vanilla extract. Beat for 30 seconds to combine.
Add flour one cup at a time and mixing on low until well combined before adding in the next cup of flour.
Scoop and roll the dough into 18 balls (about 2 tablespoons per ball), and place them on the prepared baking sheets.
Roll each ball in the sprinkles, pressing down firmly to get them to stick to the outside as much as possible. Transfer to prepared baking sheets.
Flatten the cookies by gently pressing down on the tops with a flat surface until they're about 1/4 in thick. (The bottom of a measuring cup works great for this!)
Bake for 15-20 minutes, until the bottoms of the cookies are light to golden brown.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
These cookies can be made ahead of time and frozen until ready to bake. To do so, prepare the dough as normal. Then, shape the dough into a large disc. Wrap the dough tightly with plastic wrap, and freeze for up to 3 months. When ready to use, let the dough thaw in the refrigerator overnight. Remove it from the fridge 1 hour before using.To store baked sprinkle cookies, place them in an airtight container in a cool, dry place. They can be stored this way for up to 1 week.