Prepare the Cupcake Pan: Preheat the oven to 350°F (176°C) and line a cupcake pan with cupcake liners.
Combine the Dry Ingredients: Add the flour, baking powder, baking soda, and salt to a medium bowl and whisk together; set aside. The source instructions also mention cinnamon at this stage, but no quantity is specified in the ingredient list.
Cream the Butter, Oil, and Sugars: Place the room-temperature butter, vegetable oil, light brown sugar, and sugar in a large mixer bowl and beat for about 2–3 minutes, until the mixture becomes noticeably lighter in color and fluffy. Do not shorten this creaming step.
Incorporate the Eggs: Add the eggs one at a time, mixing well after each addition. Add the vanilla extract with the second egg, scraping down the sides of the bowl as needed so everything blends evenly.
Add the First Portion of Dry Ingredients: Mix half of the dry ingredient mixture into the batter until it is mostly incorporated.
Add the Milk and Banana: Slowly mix in the milk and mashed bananas until combined. The batter may look slightly curdled at this stage; this is expected.
Finish the Batter: Add the remaining dry ingredients and mix until the batter is smooth and evenly combined. Scrape the sides of the bowl as needed, but avoid over-mixing.
Fill the Cupcake Liners: Divide the batter among the prepared liners, filling each about three-quarters full.
Bake the Cupcakes: Bake at 350°F (176°C) for 15–18 minutes, or until a toothpick inserted into the center comes out clean.
Cool the Cupcakes: Remove the cupcakes from the oven and transfer them to a cooling rack. Allow them to cool completely before filling and decorating.
Combine the Custard Base: Add the sugar and cornstarch to a medium saucepan and whisk until the cornstarch is fully dispersed and no lumps remain. Whisk in the whole milk.
Gently Thicken the Milk Mixture: Cook over medium heat, stirring continuously, until the mixture begins to thicken and just starts to simmer. Do not allow it to boil at this stage.
Temper the Egg Yolk: Add a small amount of the hot milk mixture to the lightly beaten egg yolk while whisking quickly. Then pour the tempered egg mixture back into the saucepan, whisking it into the remaining milk mixture.
Boil the Custard: Return the saucepan to the heat and bring the mixture to a light boil. When large bubbles begin popping across the surface, continue boiling for 1 minute while stirring continuously. Do not boil longer than necessary, as overcooking can cause the custard to break down instead of becoming thicker. Finish the Custard: Remove the saucepan from the heat and stir in the butter and vanilla until the mixture is smooth. Transfer the custard to a bowl. If it appears lumpy, strain it before chilling.
Cover and Chill the Custard: Press clear plastic wrap directly against the surface of the custard to prevent a film from forming. Refrigerate for 1–2 hours, or until completely cooled.
Whip the Cream: Once the custard has cooled, beat the cold heavy whipping cream with the powdered sugar until stiff peaks form.
Fold the Cream Into the Custard: Gently fold the whipped cream into the chilled custard, taking care not to deflate the whipped cream. Refrigerate the finished filling while preparing the buttercream.
Beat the Buttercream Base: Place the room-temperature butter in a large mixer bowl and beat until smooth. Add about half of the powdered sugar and mix until well combined and smooth.
Add Vanilla and Cream: Add the vanilla extract and 2 tablespoons of heavy whipping cream. Beat until smooth and fully incorporated.
Finish the Buttercream: Add the remaining powdered sugar and beat until the mixture is smooth and well combined.
Grind the Banana: Place the banana chips or freeze-dried bananas in a food processor and process until reduced to a fine powder or fine crumbs. Banana chips are firm, so they may require a little time to grind finely.
Add the Banana to the Buttercream: Add the ground banana to the buttercream and beat until evenly distributed.
Adjust the Consistency: Add additional heavy whipping cream as needed, using the remaining amount from the specified 3–4 tablespoons total, until the buttercream reaches the desired piping consistency.
Core the Cupcakes: Once the cupcakes are completely cool, use a cupcake corer or small paring knife to remove the center of each cupcake.
Fill the Cupcakes: Spoon 1 tablespoon of the chilled cream filling into the center of each cupcake.
Pipe the Buttercream: Pipe the banana buttercream over the filled cupcakes. The original recipe uses an Ateco tip 847.
Garnish and Store: Finish each cupcake with a banana chip. Place the cupcakes in an airtight container and refrigerate. The cupcakes are best eaten within 3–4 days.
