First make the ice cream. Put the cream and vanilla pod in a saucepan and heat to boiling point. Remove from the heat and leave to infuse for 20 minutes. Scrape the seeds from the pod into the cream.
Beat the egg yolks and maple syrup together in a bowl, stir in the cream and return to the pan. Heat gently, stirring, until the custard thickens to coat the back of a wooden spoon. Don't allow to boil. Leave to cool. Freeze in a plastic container, beating every hour, for 5 hours, or until frozen.
Cut the brioche slices diagonally into quarters to form triangles. Arrange, overlapping, in 4 x 250 ml (8 fl oz) baking dishes. Whisk the eggs, caster sugar, milk, cream and spice in a separate bowl. Pour over the brioche slices, pushing them down so that they are almost covered. Drizzle over the butter and scatter over the demerara sugar. Leave to soak for 30 minutes.
Set the baking dishes in a large roasting tin. Pour in enough boiling water to come halfway up the sides of the dishes. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes until set and the top is lightly golden. Serve with the ice cream.