Slice baguette into 1 ½ inch thick 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 slits and tuck chocolate into each. Place in a 9X13 in baking dish, bunching bread close together to fit.
In a bowl, whisk eggs with milk, sugar, vanilla, cinnamon and salt. Pour over bread. Turn bread over to fully soak up the egg mixture. Cover and refrigerate at least one hour or overnight.
Preheat oven to 450°.
Generously brush a large wide baking sheet with melted butter. Spaced soaked bread on baking sheet so pieces are not touching. Bake in center of oven for 8 minutes.
Remove and brush tops of bread with remaining melted butter, turn slices over.
Continue to bake until crusty and deep golden, 8 to 10 more minutes.
Place toasts on plates. Drizzle with chocolate sauce, sprinkle with powdered sugar.