Take off the screw caps on the jars. Spray with nonstick cooking spray. Set aside.
Put butter in mixer and beat at medium speed until somewhat smooth.
Pour in sugar and beat well.
Add 2 eggs.
Add: vanilla, baking powder, salt, flour, and milk.
Beat until it is mixed.
Divide your batter into six separate bowls (approximately two large ice cream scoops of batter into each bowl for even distribution).
Tint your batter the colors of the rainbow- Red, Orange, Yellow, Green, Blue, & Violet.
Using a small spoon, carefully spoon in each color of batter into layers or use a Ziploc bag and cut the tip off one corner of the bag to pipe each layer into the jars.
Place jars in a 9x13" baking pan (with rim) and fill with a ½ cup of water to create a water bath for your cake.
Bake for 25-40 minutes or until your cake springs back and a skewer poked in the center comes out clean. (length of time will depend entirely on how tall you stacked your batter!)
This should yield 8-10 rainbow jars.
Allow to cool and begin preparing your Cloud Frosting.
Cloud Frosting: Mix sugar, cream of tartar, vanilla and egg whites. Add boiling water. Beat on high until the mixture forms peaks, which should be just about seven minutes. Spoon cloud frosting on top of the cakes and finish with Rainbow Sprinkles.
Recipe by MomAdvice at http://momadvice.com/blog/2012/03/rainbow-cake-in-a-jar-with-cloud-frosting