Preheat your oven to 350 degrees.
Spray a 9×13 inch aluminum pan with non-stick spray and set aside.
In a large bowl, add your dry ingredients (flours, sugar, baking powder, salt) and stir together until mixed thoroughly.
With a handheld or stand mixer add your butter a few cubes at a time to your dry ingredients until all is added. Your batter will still be very dry.
Follow by adding in your eggs to your dry ingredients until fully incorporated.
Add in your milk and vanilla and mix for 2 minutes on medium speed until batter is fully mixed and smooth.
Lastly, fold in your rainbow sprinkles to your batter using a spatula until mixed.
Pour your batter into your 9×13 baking pan and bake 30-35 minutes depending on the heat of your oven (mine went for closer to 30 minutes). Once you see it is browned on the edges, and is firm in the middle you can remove from the oven. Use the toothpick test to see if batter is fully baked through at 30 minutes, and place back in the oven if batter is still wet. Test again with a toothpick at 35 minutes.
Allow your cake to completely cool.