Google the phrase How to bake in a mason jar and alllllll sorts of things pop up. There's tons of cute pictures and even video tutorials of baking in mason jars. You'll read mason jars can be placed in a warm water bath and then placed in an oven. You'll even see people putting jars directly in a hot oven.
But here's how you really bake in a mason jar...you don't. Yes - never, and I mean never, bake in a mason jar and here's why.
Also, cooking in glass takes significantly longer than metal as glass takes longer to heat. And because glass takes longer to heat, the middle of your dessert in jars could still be raw. And if your cake is still raw, the food isn't safe to eat.
As a precautionary measure I asked Ball Canning some advice on how to bake in a mason jar and here's their response.
I prefer using a traditional 9" round baking tin - even for cupcakes. Thing is after our cupcakes and cakes have cooked thoroughly in metal baking tins, I then use de Buyer round cookie cutters to cut cakes and cupcakes into circles.
It's a cleaner look with a perfect round circle in the mason jar. After, I've added the first layer of cake using the de Buyer cookie cutter, I add a layer of frosting using a de Buyer Pastry Syringe. And depending on the size of the mason jar, I could easily add another layer of cake and another layer of frosting.
NOTE: A safety note about air space and mason jars - If you're not refrigerating your dessert in jars like a cupcake or slice of cake, you should leave roughly a 1/4" of space between the cake and the inside of the mason jar to prevent spoilage. That's another reason why I love the de Buyer cookie cutter. It allows the cake to fall perfectly into the jar, leaving ample space around. You should also leave a little bit of air space - roughly 1/2" between the top of your cake and the lid. Because mason jars are air tight containers, it's important to have a little air space to prevent spoilage for desserts that aren't refrigerated. I'll go into more details on shelf life of mason jar desserts in a future blog post and be sure to update this post linking to that post.