Preheat oven to 325°F Grease a 2 qt square baking dish. Set aside. Place bun pieces on shallow baking sheet. Bake 7 to 8 minutes or until dry and crisp. Cool.
In medium bowl, lightly beat eggs. Stir in sweetened condensed milk, 3/4 c milk, vanilla and nutmeg; set aside.
Place bun pieces in prepared baking dish. Sprinkle with 1/2 c each of the marshmallows and chocolate pieces. Evenly pour milk mixture over all. Let stand 5 minutes. Sprinkle with crushed graham crackers. Bake, uncovered, for 35 minutes. Sprinkle with 1/4 c of the remaining marshmallows. Bake for about 5 minutes or until a knife inserted near center comes out clean.
For drizzle, in small saucepan, heat and whisk remaining marshmallows, remaining chocolate pieces and 2 T milk over low heat until melted and smooth. Drizzle over bread pudding. Cool 20 to 30 minutes before serving. Serve warm.