Butter two small gratin dishes and place the chopped brioche into the base of each dish.
Pour the custard, egg yolk, cream and Sauternes into a saucepan and warm through, whisking well. Pour half the custard mixture over the brioche.
Melt the butter in a frying pan. Chop half of the apple and add to the pan, frying over a medium heat until caramelised. Divide the apple between the gratin dishes and then pour over the rest of the custard mixture. Cover each dish with cling film and microwave on high for four minutes. Set aside and leave to rest for two minutes.
Thinly slice the other half of the apple into rings, place on a baking sheet and sprinkle with icing sugar. Using a blowtorch, carefully caramelise the top of each slice of apple. (Alternatively, you can place the apple slices under the grill to caramelise.)
To serve, top each gratin dish with the caramelised apple rings and serve immediately.