Preheat the oven to 350°F (175°C) and grease a loaf pan.
In a bowl, mash the bananas and mix with softened butter, sugar, egg, and vanilla.
In another bowl, combine flour, baking soda, and salt. Slowly add dry ingredients to the wet mixture.
Stir in brewed coffee until well combined.
Pour the batter into the prepared loaf pan.
Bake for 50 minutes, or until a toothpick comes out clean. Let cool in the pan for 10 minutes.
Meanwhile, beat together softened butter, powdered sugar, cocoa powder, coffee, vanilla, and salt for the frosting.
Spread the mocha buttercream over the cooled banana bread.