In a medium size bowl, toss together flour, baking soda, salt, and cocoa powder. Set aside.
In a separate medium size bowl with a hand mixer or the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar until light and fluffy, approximately 3 minutes. Add the egg and beat again, scraping down the sides as necessary. Add vanilla and food coloring and beat on medium speed until combined. Using a spatula, scrape down the sides and all along the bottom of your bowl to ensure coloring is distributed evenly. Add 1/4 cup of the sprinkles and beat again on medium speed until everything is incorporated.
Decrease mixer speed to low and add the flour mixture. Beat on low until dough has formed and no more pockets of flour remain. Cover bowl with plastic wrap or foil and refrigerate for at least 2 hours and up to 3 days. If chilling longer than 2 hours, allow dough to sit at room temperature for 30 minutes before rolling into balls.
When you are ready to bake your cookies, preheat the oven to 350ºF and line a baking sheet with parchment paper or a silicone baking mat.
Roll dough into balls approximately 1 Tablespoon in size. Dip the tops of the dough balls in remaining 1/4 cup of colored sprinkles, and place no more than 6 balls of dough on a baking sheet at a time. Refrigerate dough balls on baking sheet for 10 minutes.
After chilling, bake cookies immediately for 10-11 minutes. Cookies will look underdone. Remove baking sheet from oven and allow to cool for 5 minutes. Remove cookies from baking sheet and allow to cool completely on a wire rack. Cookies can be stored in an airtight container at room temperature up to 7 days or frozen for up to 2 months.
**Room temperature eggs are crucial for proper incorporation into batter. To bring eggs to room temperature quickly, place in a bowl of warm water for 10 minutes.**If using gel food coloring, use 1/2 Tablespoon.