Place rack in the upper third of the oven, and preheat oven to 350 F. Line 12 cupcake wells with foil or paper liners.
In a medium bowl, whisk flour, cocoa powder, salt, baking powder, and baking soda. Set aside. In a large bowl, whisk sugar, oil, banana, water, vinegar, and vanilla. Add the flour mixture to the avocado mixture and fold with a spatula until well combined. The batter will be so loose that you can pour it into a measuring cup to dispense into the cupcake molds.
Fill each cupcake liner until it’s about two-thirds full. Bake for 18 to 20 minutes or until a toothpick inserted into the center of the cupcake comes out clean.
Let cakes cool for 10 minutes before moving to a cooling rack.
FOR THE FROSTING
Place butter in the bowl of an electric stand mixer fitted with a paddle attachment. Beat the butter on medium speed until soft and pliable. Stop the mixer, scrape the sides of the bowl, and add the cocoa and sugar.
Turn mixer on low to slowly incorporate the dry ingredients. Add the vanilla and almond milk. Increase the mixer speed to medium or medium high and beat until frosting is light and fluffy.