Slice off a thin layer of the crust from the bottom of the bread to help the custard soak in.
Cut the loaf, hasselback style, by making long, deep vertical cuts and then horizontal cuts to create squares of about an inch and a half. Make sure you don't cut the loaf all the way to the bottom.
Place the loaf in a baking dish.
Make the custard by placing all ingredients in a blender--milk, banana, cornstarch, brandy or whiskey, melted vegan butter, vanilla, maple syrup and sugar, and nutmeg. Blitz until very smooth.
Pour the custard over the loaf, separating the squares gently with your fingers to make sure you get inside every nook and cranny. Set aside for at least half an hour or cover with cling wrap and let it stand overnight in the fridge.
Preheat the oven to 350 degrees Fahrenheit. Spoon any custard that has pooled at the bottom of the pan back over the bread.
Bake the French toast for 45 minutes or until the top is crusty. Let stand 10 minutes, then remove to a platter and serve, dusted with powdered sugar.