Preheat the oven to 350°F (180°C) and line two baking sheets with parchment paper. Set aside.

In a medium bowl, sift together the flour, baking soda, and salt. Set aside.

In a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar, and powdered sugar on medium-low speed for 4 minutes. Mix in the egg and vanilla extract until just combined. Add the flour mixture and mix until just incorporated. Gently fold in the sprinkles, being careful not to overmix.

Shape the dough into balls, about 1.5 tablespoons each (an ice cream scoop works well for this), and arrange them on the prepared baking sheets, spacing them 2 inches apart.

Bake for 11-12 minutes, until the edges are just beginning to brown while the centers remain soft—avoid overbaking. Let the cookies cool on the baking sheet for 5-10 minutes before carefully transferring them to a wire rack to cool completely.

Store in an airtight container at room temperature for up to 3 days, or freeze for up to 2 months.