Preheat the oven to 375. Prepare two baking sheets by lining with parchment paper.
Cream butter, shortening, and sugar in stand mixer for about 3 minutes, until creamy and fluffy. Scrape down sides and combine again. Add egg and vanilla, mix on medium until combine.
Combine the flour, salt, baking soda, and baking powder in a separate mixing bowl. On low speed, slowly add the flour mixture to butter mixture until combined.
Roll into tablespoon-sized balls and roll in the rainbow sprinkles. Place on parchment-lined baking sheets. Gently flatten with a glass. Sprinkle with extra sprinkles in desired.
Bake for 10-12 minutes, or until set. They will be light in color but the edges should be starting to turn golden.
Notes
The cookies may appear slightly underdone but don't overbake or they'll be crisp instead of chewy. Fully baked and cooled cookies freeze very well, as do unbaked balls of cookie dough.