((Reese Cake in a Jar)) Step 1. Preheat oven to 350F Step 2. Spray jars with canola oil Step 3. Make chocolate cake recipe Step 4. Scoop 3 or 4 tablespoons of your chocolate cupcake recipe in each jar Step 5. Add 2 tablespoons of peanut butter over mix. Step 6. Add 2-3 more tablespoons of mix. Step 7. Bake around 25 minutes.