Preheat the oven to 350°F. Line two muffins pans with paper cups.
In a small bowl, combine the mashed bananas with the lemon juice. Set aside.
In a separate large bowl, whisk together the flour, baking soda and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy, about 2 minutes, then beat in the eggs one at a time, beating between each addition.
Beat in the vanilla extract.
Beat in the flour mixture alternately with the buttermilk.
Fold in the mashed bananas then portion out the batter into the muffin cups so that they are 3/4 full. (This recipe yields about 30 cupcakes.)
Bake the cupcakes for 18 to 22 minutes until a toothpick inserted comes out clean.
Remove the cupcakes from the oven and set them aside to cool while you make the frosting.