Preheat your oven to 350°F (175°C) and set aside a cupcake pan lined with liners. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until thoroughly combined. In another bowl, mix the milk, egg, vegetable oil, mashed banana, and vanilla extract until smooth. Gradually pour the wet ingredients into the dry mix, stirring gently to combine.

Carefully fill each cupcake liner about three-quarters full with the batter. Place the cupcake pan in the preheated oven and bake for 20 minutes or until the tops spring back when gently pressed. Allow the cupcakes to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.

While waiting for your cupcakes to cool, prepare the buttercream frosting. In a mixing bowl, beat the unsalted butter with an electric mixer on medium speed for about 3-5 minutes or until light and fluffy. Gradually add the powdered sugar and heavy cream, mixing until smooth and creamy.

To create a chocolate ganache, place the semisweet chocolate chips in a microwave-safe bowl. Pour the heavy cream over the chocolate and microwave in 30-second intervals, stirring until smooth and glossy.

Once the cupcakes have cooled completely, use a piping bag to generously pipe the buttercream frosting onto each cupcake. Chill the frosted cupcakes for about 30 minutes to set the frosting. After chilling, drizzle the chocolate ganache over the top, and finish with rainbow sprinkles and a maraschino cherry.