Preparation: Begin by preheating your oven to 170ºC (350ºF). Use 2 tablespoons of butter to lightly grease a 23x33cm (9x13 inch) baking dish.
Maple Butter Base: In a small saucepan, combine the remaining butter, brown sugar, and maple syrup. Whisk them together over medium heat until they're well combined, and pour this maple butter into the bottom of your prepared baking dish.
Brioche Layering: Slice your brioche bread into triangles and arrange them in the dish, cut side down, with the crusts facing upwards. This positioning allows for optimal custard absorption.
Custard: For the custard, whisk together eggs, milk, and cinnamon until smooth. Gently pour this mixture over the arranged brioche, ensuring each piece is evenly coated. Allow the bread to soak for at least 30 minutes, or for an even richer flavour, let it rest overnight in the fridge. Press down lightly on the bread to ensure it's fully submerged and even in the dish; the bread may rise slightly as it absorbs the custard.
Baking: Bake in the preheated oven for 30-35 minutes. If the edges begin to brown too quickly, cover the dish with foil and continue to bake until the custard sets and the top is golden and crispy.
Serving: After removing from the oven, let the bake stand for a few minutes. In the meantime, whip the double cream (heavy cream) until light. Dollop the cream onto the warm French toast bake; it will start to melt, similar to ice cream. Finish by spooning raspberry compote over the top and serve warm.