Prepare pie crust well in advance.
Place castor sugar, egg yolks, milk, cream, cornflour and vanilla paste into the Thermomix bowl and cook 8 minutes/80°C/speed 4.
Add 40 grams butter and cook 2 minutes/100°C/ speed 4.
Remove from the Thermomix bowl immediately and cover with a baking paper pressed to the surface of the custard until cold, then transfer to the fridge.
Heat a frying pan to a medium–high heat and melt the 20 grams of butter with the dark brown sugar until melted and combined.
Toss the sliced bananas in the frying pan until well coated with the caramel. Add the salt and cook for around 1 minute. Remove bananas and allow to cool completely.
When ready to assemble and serve the pie, place the caramelised bananas into the base of the prepared pie crust, reserving a couple of slices for garnish.
Stir the vanilla custard mixture (you may need to re-blend it for a few seconds/speed 4 to achieve a pourable consistency) and pour it onto the bananas. Refrigerate until ready to serve.
Top with plenty of whipped cream and the reserved caramelised banana slices. Serve within a few hours for best results.