In a medium bowl, whisk together the egg (or egg yolks), sugar, cornstarch, and salt for 1-2 minutes until smooth.

In a medium saucepan, heat the milk over medium heat until it just comes to a simmer.

Temper the eggs by streaming half of the milk into the egg mixture while whisking.

Dump the whole mixture back into the saucepan and cook over medium-low heat, whisking continuously until thickened (1-3 minutes).

Once thickened, whisk aggressively for an additional 30-60 seconds and then remove from heat.

Pour into a bowl and whisk in the butter and vanilla until smooth.

Preheat the oven to 350F/177C convection (325F/163C conventional) and line 13 muffin tins with cupcake liners.

In a medium bowl, whisk together the cake flour, baking powder, salt, and sugar until evenly distributed.

Add the oil and butter to the dry ingredients and beat together until combined.

Add the egg and vanilla extract, mixing until fully combined.

Add the sour cream, turn the mixer to low speed, and stream in the milk until the batter is smooth.

Place a vanilla wafer cookie on the bottom of each cupcake liner and fill 3/4 full with batter (45-50 grams each).

Bake for about 18-20 minutes or until a toothpick inserted comes out clean.

Let the cupcakes cool in the pan for 10 minutes before moving to a cooling rack to cool completely.

Sift the powdered sugar and salt into the bowl of a stand mixer, then pour in the heavy whipping cream and vanilla.

Whip until thickened, then continue until stiff peaks form.

Assemble by spreading a layer of pastry cream on top of the first cupcake layer, then add banana slices and crushed wafers, followed by whipped cream.

Repeat layers, finishing with whipped cream on top. Decorate with banana slices and wafer cookies.

Refrigerate for at least a few hours to set. Serve cold or slightly chilled.