Preheat your oven to 350°F. Line a 12-cup muffin tin with paper liners.

In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda, and fine sea salt. Set aside.

In a large bowl, using an electric mixer, beat the softened butter and granulated sugar on medium speed until light and fluffy, about 3 minutes. Add the mashed banana, buttermilk, and eggs, and mix until well combined. Gradually add the dry ingredients, mixing on low just until incorporated. Divide the batter evenly among the liners (an ice cream scoop works well). Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cupcakes from the tin and let them cool completely on a wire rack before frosting.

In a large bowl, beat the softened butter, powdered sugar, and vanilla extract together with an electric mixer until smooth and creamy. Add the heavy cream or milk, 1 tablespoon at a time, beating until you reach a spreadable consistency. Transfer the frosting to a piping bag fitted with a large round tip, and pipe a generous swirl onto each cooled cupcake.

Place the chocolate chips in a small heatproof bowl. In a separate microwave-safe bowl, heat the heavy whipping cream for about 45 seconds, until hot but not boiling. Pour the hot cream over the chocolate chips and let it sit for 2 minutes without stirring. Then stir gently until the chocolate is completely melted and the ganache is smooth. Let the ganache cool for a few minutes until it thickens slightly, then spoon it over the frosting on each cupcake.

While the ganache is still tacky, sprinkle rainbow sprinkles on top. Pipe a small dollop of whipped cream on the center of each cupcake, then top with a maraschino cherry. Serve immediately, or refrigerate until ready to serve. For best results, add the whipped cream and cherry just before serving to keep them fresh.