Preheat oven to 350 degrees Fahrenheit.
Make your flax "egg" by whisking 1 tablespoon of flaxseed meal with 2 tablespoons of water and put it in the refrigerator to set.
Melt the vegan butter in the microwave in 20-second intervals and beat in sugar.
Add the flour, baking powder, and salt and mix until combined.
With the mixer on low, beat in vanilla and flax "egg".
Use a spatula to smooth out the dough and fold in 1 tablespoon of rainbow sprinkles. Add the remaining sprinkles to a shallow bowl or plate.
Use clean hands to form dough into balls, using about 1 tablespoon of dough at a time. Take each ball of dough and roll half of it in the bowl of sprinkles to coat them. Set in the refrigerator for 15 minutes.
Bake for 13-15 minutes and allow to cool before serving.