Slice baguette into 1 1/2 inch rounds
Using a serrated knife cut through the middle of the bottom crusty side of each round to make a slit large enough to hold a square of chocolate. Widen slit and add a piece of chocolate into each.
Place into a 9 x 13 in pan bunching close together.
In a bowl whisk egg, milk, sugar, vanilla, cinnamon and salt. Pour over bread.
Turn bread over to fully soak up egg mixture.
Cover and refrigerate for at least 1 hour or overnight.
Preheat oven to 450ºF.
Generously brush a large wide baking sheet with butter. Place soaked bread onto sheet so pieces are not touching
Bake for 8 minutes
Remove from oven and brush top of bread with butter.Turn slices over and continue to bake until golden brown, 8-10 minutes
Place on plates and sprinkle with icing sugar.