I do not recommend doing it the way that I did. I split up the cake batter into two different 9x13 pans, baked and cooled the cakes, and then used the jars upside down like a cookie cutter to get a round piece of cake into the jar. Looking back, it would have been much easier to just bake a normal cupcake, unwrap it, and put it in the jar. There wouldn't have been any waste that way.
Then I just frosted them with a piping bag like I normally would have, and topped them with a few sprinkles. TaDa: Cute little cupcake to-go! I plan on sending them to school with the kiddos tomorrow, I'm sure the teachers will appreciate a sweet treat. These would be great for birthday parties too!