In a bowl mix milk, eggs, sugar, cinnamon and vanilla.
Grease a baking tray with fairly deep sides with butter and layer the bread slices in a scale pattern. Pour over the milk and egg and leave covered overnight in the fridge.
The next day, preheat oven to 350°F/180°C.
Make a syrup by whisking together butter, maple syrup and demerara sugar and pour evenly over the french toast mixture, then bake for 20 minutes until the top is slightly caramelised. Serve when cool with ice cream.