Place the butter, chocolate, sugar and milk in a small pan and heat gently until the sugar has dissolved and the chocolate has melted. Set aside to cool a little.
Whisk the eggs in a medium bowl, then gradually add the chocolate mixture, whisking continuously.
Grease a shallow 18 x 23 cm (7 x 9 inch) ovenproof dish. Soak each slice of brioche in the chocolate mixture, then layer the slices in the dish.
Pour over any excess liquid, then bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until risen and just set. Dust with icing sugar and serve with some reduced-fat crème fraîche on the side.