Cupcakes: Preheat oven to 350 degrees. Line a cupcake tray with paper liners. In a separate bowl, blend flour, baking powder and salt. Set aside. In a large mixing bowl, beat together butter, sugar and vanilla extract, until light and fluffy. Add eggs, one at a time, beating well after each addition. Set aside. In a small bowl, combine mashed banana and buttermilk. Add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture; mix after each addition. Fill the cupcake liners three-quarters full with cupcake batter. Bake the cupcakes for 20 to 25 minutes, or until they’re golden brown and cooked through. Cool the cupcakes completely.
Banana cream filling: In a separate bowl, blend flour, sugar and salt. In a separate saucepan, heat milk over medium heat till hot, but not boiling. Slowly stir in the flour mixture and simmer or 2 minutes, stirring constantly. Remove from the heat and stir in the egg yolk. Heat again for 2-3 minutes, stirring continuously. Remove from the heat, and let cool. Stir in mashed banana and lemon juice, until well combined.
Vanilla frosting: In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, until light and fluffy. Gradually add powdered sugar a little at the time, beating on medium speed until well mixed. It takes about 2 minutes or so. Beat in the heavy cream, vanilla and lemon juice. Continue beating until the frosting reaches the desired consistency, about 4 minutes.
Chocolate ganache: Heat chopped choclate in a small saucepan over medium heat until chocolate is melted. Set aside. In a small saucepan, heat heavy cream until just about boiling, stirring constantly. Remove from the heat and pour hot cream over chocolate. Stir to blend well. Allow to cool.
To assemble the cupcakes: Cut out the center of each cupcake with a sharp knife. Fill the cupcakes with the banana cream filling. Transfer the frosting to a piping bag fitted with a decorative tip. Pipe frosting into a swirl on each cupcake. Drizzle each cupcake with chocolate ganache and garnish with a Maraschino cherry.