How to Make It
In a medium saucepan, combine the milk with the vanilla extract and banana and bring to a simmer. Remove from the heat and let stand at room temperature for 1 hour (or refrigerate overnight). Return the mixture to a simmer, then strain through a sieve into a heatproof measuring cup; discard any solids.
In a large bowl, combine the egg, egg yolk, sugar, cornstarch and salt and whisk until thickened and smooth. While whisking constantly, gradually whisk in 1/4 cup of the hot milk to temper the eggs, then gradually whisk in the remaining milk until combined.
Pour the egg-and-milk mixture into the saucepan and cook the custard over low heat, whisking constantly, until thickened, about 8 minutes. Strain the custard through a sieve into a bowl. Press a piece of plastic wrap directly on the surface of the custard and refrigerate until chilled, at least 1 hour.
Preheat the oven to 350° and line a 12-cup muffin tin with paper liners. In a medium bowl, whisk the flour with the baking powder and salt.
In a large bowl, using a handheld electric mixer, beat the butter with the peanut butter, granulated sugar and brown sugar at medium-high speed until fluffy, about 3 minutes. Beat in the eggs, one at a time, then beat in the vanilla. At low speed, beat in half the flour mixture, then all the milk, followed by the remaining flour mixture.
Fill the prepared muffin cups with the batter until almost full and bake for about 20 minutes, until a toothpick inserted in the center of the cupcakes comes out clean. Transfer the cakes to a wire rack and let cool completely.
Put the peanut butter chips in a large heatproof bowl. In a small saucepan, bring the heavy cream to a simmer, then pour it over the peanut butter chips and let stand for 5 minutes. Add a pinch of salt and whisk the frosting until smooth; let cool, stirring occasionally, until spreadable.
Transfer the banana cream into a piping bag fitted with a large round tip. Insert the tip into the top of each cupcake and fill with some of the cream. Frost the cupcakes and garnish with peanuts.