Begin by preheating your oven to 350 degrees Fahrenheit. Lightly grease (with vegan butter) AND line with parchment paper the bottoms of TWO 9-inch or 8-inch cake pans (for thicker layers) or THREE 6-inch cake pans.
In a bowl, whisk together the milk and apple cider vinegar and set aside for 5-10 minutes until everything "activates" to create the dairy-free "buttermilk".
In a large bowl, whisk together the flour, baking powder, baking soda, and salt until combined.
In a bowl, add the milk-acv mixture, melted vegan butter, sugar, and using a hand-held mixer (on high-speed), mix everything together until combined and the mixture becomes wet.
On low-speed, add in the dry ingredients and continue mixing until the batter is combined, smooth, and thick, about 3-4 minutes. NOTE: The batter should be thick, yet slowly run off the whisk once lifted.
Fold in the rainbow sprinkles.
Divide the batter evenly into each of the cake pans and bake for 30-35 minutes, or until a toothpick or butter knife comes out clean when tested in the centers.
Remove cakes from the oven and let them rest in the pan for 10 minutes or so before removing the cakes and placing them onto a cooling rack to fully cool. NOTE: Let them cool for at least 1-2 hours in a cool place.