Heat sugar, butter, and corn syrup in a saucepan over medium heat until well-blended, stirring frequently.
Pour the sugar mixture into a greased 9x13 inch baking pan.
Arrange bread slices in a single layer on top.
In a separate bowl, whisk hot chocolate, eggs, vanilla, and salt in a bowl until well-blended.
Pour the egg mixture over the bread.
Chill, covered with foil, 8-10 hours (overnight).
Bake, covered, at 350 degrees for approx. 45 minutes. Remove foil after first 15 minutes.
Prepare glaze by combining powdered sugar and 2 Tbsp. hot chocolate.
Drizzle baked french toast with glaze and top with marshmallow bits and mini chocolate chips.