In a large mixing bowl, beat together butter and sugars until light and fluffy. Add the egg and vanilla and mix until incorporated. Add flour, baking soda, salt, and cream of tartar, mix just until combined. Add sprinkles and stir.
Scoop dough into balls and place on ungreased cookie sheets about 2 inches apart. Bake in preheated oven for about 10 minutes, until puffy and lightly browned. Remove from oven and let cool on cookie sheets for about 5 minutes before removing to wire racks to cool completely.
Notes
This recipe makes approximately 2 dozen cookies if using a 1.5 TB cookie scoop.I highly recommend these rainbow sprinkles.