In a large bowl add flour, baking powder and sprinkles. Whisk together and set aside.
In the bowl of an electric mixer add butter and beat until smooth. Add sugar and blend until fully combined and fluffy. Add egg and vanilla and blend.
While mixing, slowly pour in the bowl of flour. Mix until dough forms and starts pulling away from the sides of the bowl.
Scoop dough out onto a work surface and form into two large balls. Spread out a piece of plastic wrap on your work surface and set one ball of dough in the center. Lay a second piece of plastic wrap on top and roll the dough to about 1/3" thick. Carefully pick up the slab of dough, still enclosed in the plastic wrap, and place slab on a large baking sheet. Freeze slab of dough for about 15 minutes, or until firm.
Use a knife or bench scraper to cut the dough into small squares. Mine were about 3/4" x 3/4".
Preheat oven to 350*F. and arrange cookies about 1" apart on a baking sheet lined with parchment paper or a Silpat type liner.
Bake cookies for 8-10 minutes.
Cool cookies on the baking sheet for 1-2 minutes, then move to a cooling rack. Once fully cool, store cookies in a sealed container until ready to decorate. Cookies remain fresh (when stored well) for at least a week.
Feel free to use sprinkles of your choice. Avoid re-rolling the dough too many times, as the heat from your hands and moisture in the dough will cause the colors in the sprinkles to bleed.