Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a large mixing bowl, cream the room temperature unsalted butter and granulated sugar together on medium speed for 2-3 minutes until fluffy.

Add large eggs and pure vanilla extract, mixing on low speed until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, baking soda, cream of tartar, and salt. Gradually add to the wet ingredients and mix until just combined.

Fold in the rainbow sprinkles using a rubber spatula or wooden spoon until evenly distributed.

Divide the dough into four portions, color three with gel colors, and leave one uncolored. Knead gently.

Lightly swirl the colored portions together to create a marbled effect.

Scoop out small portions, roll into balls, and coat in granulated sugar or extra sprinkles. Place on the baking sheet spaced 2 inches apart.

Bake for 10-12 minutes until edges are lightly golden and centers are soft.

Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.