01 -
Preheat your oven to 350°F. Line a large cookie sheet with parchment paper or a silicone baking mat to prevent sticking.
02 -
In a large bowl, beat the softened butter and sugar together on medium speed until light and fluffy, about 2-3 minutes. Add the eggs and vanilla, then continue mixing until everything is well combined and smooth.
03 -
In a separate bowl, whisk together the flour, baking soda, cream of tartar, and salt. Gradually add this dry mixture to your butter mixture, mixing on low speed until just combined and a dough forms. Stir in the rainbow sprinkles until evenly distributed.
04 -
Divide the dough into 4 equal portions. Leave one portion uncolored. Add pink gel coloring to the second portion, royal blue to the third, and violet to the fourth. Gently work each color into its dough portion using a spatula or your hands (wearing food-safe gloves is recommended).
05 -
Place chunks of all four colored doughs into a large bowl. Very gently fold them together just a few times to create a marbled, tie-dye effect. Be careful not to overmix, or you'll lose the distinct colors.
06 -
Use a medium cookie scoop to portion the dough into balls (about 2 tablespoons each). Roll each ball in additional sprinkles or granulated sugar if desired. Place the cookies about 2 inches apart on your prepared baking sheet.
07 -
Bake in your preheated oven for 10-12 minutes. The edges should be just barely set, while the centers still look slightly underdone (they'll continue cooking on the hot baking sheet).
08 -
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. The cookies will firm up as they cool while staying soft and chewy inside.