For the crème brûlées: Preheat the oven to 150°C (130° fan) | 300F | gas 2.
Warm the cream with the milk and split vanilla bean (and seeds) in a heavy-based saucepan set over a medium heat. Remove from the heat as it starts to come to the boil.
Beat the egg yolks with the sugar in a large, heatproof mixing bowl until pale and thick, about 2-3 minutes.
Whisk the warmed cream and milk into the egg yolk mixture in a slow, steady stream. Fine strain the custard back into a saucepan and discard the vanilla bean.
Cook over a low heat, stirring constantly, until slightly thickened, about 3-4 minutes. Pour into a bowl and set aside to cool.
Place four individual ceramic ramekins into a roasting pan or baking dish. Arrange blueberries and raspberries in each ramekin.
Divide the custard mixture between them, pouring slowly to prevent air bubbles forming. Fill the roasting pan with enough hot water so that it comes about halfway up the sides of the baking dishes.
Bake the custards for about 50-60 minutes until set with a slight wobble when tapped.
Remove from the oven and lift the custards out of the water bath. Leave to cool slightly before covering and chilling overnight until set.
To serve: When ready to serve, top each brûlée with 1 tbsp soft brown sugar in an even layer. Use a chef's blowtorch to caramelize the sugar.
Serve on plates garnished with blueberries, raspberries and mint sprigs.