Preheat oven to 350°F (175°C). Line a cupcake pan with liners.
In a small bowl, mix mashed bananas with lemon juice, set aside.
In a medium bowl, mix flour, baking soda, and salt, set aside.
In a large bowl, cream ½ cup butter, granulated sugar, and brown sugar until light and fluffy.
Beat in eggs one at a time, then stir in vanilla.
Beat in the flour mixture alternately with the buttermilk.
Fold in banana mixture.
Fill cupcake liners about ¾ full.
Bake in preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let cupcakes cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
For the frosting, beat peanut butter and ¼ cup butter until creamy.
Mix in cocoa powder and powdered sugar alternately with milk until smooth and spreadable.
Frost cooled cupcakes.