Make the crust: Mix together the pretzels, butter and sugar and line the base and sides of your pie dish.
Make the custard: Pour the milk into a pan and add the split vanilla pod and its seeds. Bring the milk mixture to the boil, then remove from the heat. Whisk the sugar, egg yolks and cornflour together in a large bowl. Pour out a little of the hot milk onto the egg mixture, whisking continuously. Whisk in the rest of the hot milk until well-combined, then return to the pan. Cook the mixture over a gentle heat, stirring continuously, until the mixture becomes thick. It will just come to the boil. Add the dulche de leche and stir until thoroughly combined.
Pour over the base then leave to set in the fridge for a minimum of 3 hours.
To serve: Cut the banana into horizontal slices, then lay over the top of the custard. Sprinkle with the sugar and brulle with a blowtorch until caramelised.