How to Make Cake in a Jar
To make cake-in-a-jar using the microwave, follow the instructions on a box of chocolate cake or brownie mix, but make sure to add 1 extra egg. When the ingredients are incorporated, fill mason jars halfway with batter. Press 2 chocolate squares into the center of each jar, then cover with more batter and microwave for 1 minute. If you make more than 1 cake at a time, add an extra 30 seconds per cake. Serve the cakes straight from the jars and garnish with whipped cream or ice cream.
Preheat your oven to 350°F (177°C) and prep the jars. Coat the insides of the jars with nonstick cooking spray or vegetable shortening. Make sure that the jars you are using have wide mouths and straight walls. Set the lids aside for now. Beat the butter and sugar. Place the butter into a medium-sized mixing bowl. Add the sugar, then beat the two together. Keep beating them until the butter turns light and fluffy. You can do this by hand, or speed things up with a mixer or food processor. Add the eggs gradually, then the vanilla extract. Add in the first egg, and beat the batter well. Add in the second egg, and beat the batter again until no streaks remain. Stir in the vanilla extract last. Combine the flour, baking powder, and salt in a separate bowl. Pour the flour into a small mixing bowl. Add the baking powder and the salt. Whisk the three together until they are evenly combined and no clumps remain. Alternate between adding the flour mixture and the half-and-half into the butter mixture. Pour half of the flour mixture into the butter mixture. Whisk it in well, then stir in all of the half-and-half. Whisk in the rest of the flour mixture. Fill the jars halfway with batter. You can pour the batter into the jas straight from the bowl, or you can ladle them out using an ice cream scoop or soup ladle. Be sure to wipe the mouths of the jars clean when you are done. This ensures that they seal properly later on. Set the jars onto a baking sheet, then bake them for 25 minutes. The baking sheet will make it easier to take the jars in and out of the oven. It will also keep them from tipping over. The cakes are done when they spring back slightly when touched in the center. Make sure that the jars are not touching on the baking sheet. If you want to seal the jars later, boil the lids in a large saucepan while the cakes are baking. Seal the cakes, if desired. Let the cakes cool for 1 to 2 minutes, then seal them with the lids. The heat from the cakes will cause the lids to automatically seal when you put them on. You can skip this step if you plan on serving all of the cakes on the same day. Serve the cakes straight from the jars. If you plan on serving the cakes on the same day, be sure to let them cool first, about 10 to 15 minutes. Top them off with some chocolate sauce, whipped cream, or anything else you'd enjoy on a cake. Whisk the chocolate cake or brownie mix. Pour the mix into a large mixing bowl, then whisk it to remove any lumps or clumps. This recipe will give you a chocolate molten lava cake. If you don't like chocolate cake, use white cake mix to make white cake instead. If you have a different mug cake recipe that you prefer, you can use that instead. Combine the water, oil, and eggs in a separate bowl. Take a look at the box you cake or brownie mix came in. Measure out the water, oil, and eggs called for on the box, and place them into a separate mixing bowl. Add one extra egg, then whisk everything together. You are adding an extra egg because this will make your cake richer and taste more homemade. Most cake mixes will call for 1 cup (240 milliliters) water, ⅓ cup (80 milliliters) vegetable oil, and 3 eggs. Most brownie mixes will call for: ¼ cup (60 milliliters) water, ¼ cup (60 milliliters) oil, and 1 egg. Stir the wet mixture into the dry mixture. Pour the whisked water, oil, and eggs into the dry cake or brownie mix. Stir everything together until combined and no lumps or streaks remain. Fill the jars halfway with butter. You can try to pour the batter straight from the bowl, or you can ladle it out using an ice cream scoop or soup ladle. You will have enough batter to fill about six 8-ounce (240-milliliter) mason jars. Set the lids aside for now. Do not over-fill the jars. If you fill them more than halfway, the batter may rise too much and overflow. Press two chocolate squares into the center of each cake. Choose a rich, soft, dark chocolate. Press two squares into the center of each mason jar, then cover them with more batter. If you are making a different type of cake, you can skip this step. Cook the mason jars in the microwave for 1 minute. You can cook up to four cakes at one time. If you choose to cook more than one cake, add 30 seconds for each additional cake. This means that if you are cooking four cakes, you should cook them for a total of 2 minutes and 30 seconds. The cakes will puff up as they cook, and deflate when you stop the microwave. Place a paper towel or paper plate under the jar. This way, if there are any spills, they will be easier to clean up. Every microwave is different, so the cooking times on your cake might be a little different. Serve the cakes straight from the jars. If you want to, you can garnish them with a dollop of whipped cream or a scoop of vanilla ice cream. Cover any leftover jars with the lids. The heat from the jars will automatically cause the lids to seal. Let the cakes cool down to room temperature before putting them into the fridge. 
Preheat your oven to 350°F (177°C) and prep the jars. Beat the butter and sugar. Add the eggs gradually, then the vanilla extract. Combine the flour, baking powder, and salt in a separate bowl. Alternate between adding the flour mixture and the half-and-half into the butter mixture. Fill the jars halfway with batter. Set the jars onto a baking sheet, then bake them for 25 minutes. Seal the cakes, if desired. Serve the cakes straight from the jars. Whisk the chocolate cake or brownie mix. Combine the water, oil, and eggs in a separate bowl. Stir the wet mixture into the dry mixture. Fill the jars halfway with butter. Press two chocolate squares into the center of each cake. Cook the mason jars in the microwave for 1 minute. Serve the cakes straight from the jars. 