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 comes to a simmer.

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

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

Once thickened, whisk for another 30-60 seconds, then remove from heat and mix in the butter and vanilla.

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

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

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

Add the sour cream and mix on low speed, gradually streaming in the milk until smooth.

Place a vanilla wafer cookie at the bottom of each cupcake liner and fill with batter (45-50 grams each).

Bake for 18-20 minutes until a toothpick comes out clean. Cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.

Sift powdered sugar and salt into a mixer bowl. Pour in heavy whipping cream and vanilla, mixing on medium speed until it starts to thicken.

Increase to medium-high and whip until stiff peaks form, being careful not to over-mix.

Slice bananas and toss with lemon juice to prevent browning.

On a flat surface, spread a layer of pastry cream on the first cupcake layer, add banana slices, and a layer of crushed vanilla wafers.

Top with whipped cream, add another layer of cake, and repeat until finished.

Spread remaining whipped cream on top and decorate with bananas and wafer cookies.

Refrigerate for at least a few hours to set before serving.