In a large mixing bowl, cream the butter and the sugar on high until the mixture is fluffy and there are no lumps; break apart any chunks with a fork and scrape down the sides of the bowl.
Add the vanilla extract and the sour cream and mix on low, then add the eggs and mix on low until just incorporated; do not overmix.
Add the baking soda, cornstarch, salt, and just one cup of the all purpose flour, then mix on low. Slowly add the rest of the flour one cup at a time and mix until there are no more streaks of flour in the dough.
Divide the dough evenly into three bowls and add a few drops of gel food coloring to each bowl, mixing with a spatula or spoon until you reach the desired color (cookies will lighten slightly when baked). Cover the bowls with plastic wrap and refrigerate to chill for at least 30 minutes; the dough will be sticky but firmer after chilling.
Once the dough has chilled, preheat the oven to 375°F/190°C and line two baking sheets with parchment paper or a silicone mat.
Scoop about half a tablespoon of each colored dough and roll into individual balls (about 1 inch in diameter). Grab one ball of each color, press them together, and carefully roll them into one larger marble ball.
Place each combined ball about 4 inches apart on the prepared baking sheets and lightly flatten them into a small cookie shape with your fingers to allow for spreading.
Decorate each cookie with white chocolate chips and sprinkles, then bake for 8-10 minutes, checking for doneness after 8 minutes; they should still be a little soft when removed. Allow the cookies to cool in the pan for 10 minutes before transferring.