Preheat oven to 350°F.
Grease an 8" x 6" rectangular baking dish and set aside. Or, if you want to make a big, full-sized cake, double the recipe and use a normal cake pan.
Mix flax and water together in a small bowl and set aside.
Melt chocolate by microwaving in a microwave-safe bowl for 30 seconds and then in 10-second intervals until melted. Mix between each interval.
Once chocolate is melted, pour in plant milk and heat again for 30 seconds. Mix together until chocolate and milk have combined.
Sift together dry ingredients (up to salt) in a large bowl.
Mash banana or pour applesauce into a bowl, then add peanut butter. *Note, your peanut butter should be runny, so if it's too solid then microwave until soft.
Add in the chocolate/milk mixture, flax egg, lemon juice, and mix together until combined.
Pour the wet ingredients into the dry and gently mix until combined. Be careful not to overmix.
Bake at 350°F for 25-30 minutes, or until a toothpick inserted comes out clean.