Heat your oven to 350F and line the bottom of an 8X8-inch baking pan with parchment paper or aluminum foil. Spray with a light coating of vegetable spray.
Place the vegan butter in a microwave-safe mixing bowl and heat in the microwave for about a minute, until melted.
Add the flax egg, brown sugar, mashed banana, and vanilla and stir until well incorporated.
In a separate, small bowl combine the flour and salt and give it a whisk to combine. Add the flour to the banana batter. Stir to combine.
Pour the batter into your prepared pan and bake for 25 minutes, until a toothpick inserted into the middle comes out clean.
In the meantime prepare your Creamy Peanut Butter Frosting (see below).
Allow bars to cool for about 10-15 minutes before removing from the pan and adding the frosting.
For the Creamy Peanut Butter Frosting:
Combine the peanut butter and coconut oil in a medium-size bowl. Stir to combine. If your coconut oil is too firm, it can be helpful to heat it up by placing it in the microwave for about 11 seconds. It doesn't take long!
Add the powdered sugar and stir until you have a crumbly mixture. Add the plant-based milk one tablespoon at a time until you reach a desired, spreadable consistency.
Notes
You can sub coconut oil for the vegan butter. You can sub whole wheat pastry flour for all-purpose flour.You can sub a gluten-free baking flour for the flour to make this gluten-free.