Preheat the waffle iron per your manufacturer's directions.

In a large bowl, add the dry cake mix.

In a liquid measuring cup, combine the water, oil, eggs, and vanilla and whisk until fully combined.

Pour the wet ingredients over the dry ingredients and stir with a rubber spatula or a wooden spoon until combined, but some lumps are ok.

Add the rainbow sprinkles (if using), and stir gently until the batter is smooth and the sprinkles are evenly distributed.

Spray the hot waffle iron with cooking spray and pour 1/2 cup (see notes) of batter into the center of the waffle maker. Spread it slightly so that it's evenly distributed, but do not push the batter all the way out to the edges

Close the lid of the waffle maker and cook until golden brown, per the recommended time listed on your waffle maker's instructions.

Once the waffle is cooked, carefully remove it from the waffle iron and serve immediately or set it on a wire rack in a warm oven to serve later.

Repeat the above process with the remaining cake mix batter, spraying the hot waffle iron in between each round of waffles.