For the cupcakes: Preheat the oven to 325 degrees F and line muffin pans with paper liners.
In medium bowl, combine the bananas and lemon juice. Sift together the flour, baking soda, salt and cinnamon.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter and sugar until light and fluffy. Beat in the eggs and vanilla.
Add the buttermilk and flour mixture alternately. Fill the liners about three-quarters of the way full and bake, 17 to 19 minutes.
For the filling: In a small saucepan, heat the heavy cream until it just reaches a boil.
Pour into a medium mixing bowl over the bittersweet chocolate and let sit for 2 minutes. Stir the mixture and let come to room temperature.
For the pastry cream: In medium saucepan over medium heat, combine 2 1/2 cups of the milk, the sugar and salt. Cook until all the sugar dissolves, and then add the vanilla.
Combine the remaining 1/2 cup milk with the cornstarch, flour and egg yolks.
Whisk some of the hot milk mixture into the egg yolk mixture to be sure to not scramble the eggs. When tempered, add the yolk mixture back to the saucepan.
Keep stirring with a whisk until the mixture is thick enough to coat the back of a wooden spoon. Strain through a mesh strainer to remove any lumps. Stir in the peanut butter and butter. Cover the top with plastic wrap and refrigerate until completely cool. Fold in the whipped cream.
To assemble: Core out the cupcakes and fill with pastry cream. Dip the tops of the cupcakes into chocolate ganache.
