1. Heat oven to 350 degree F. Combine graham-cracker crumbs, melted butter and marmalade in a medium bowl. Press mixture onto bottom and 1 inch up sides of a 9-inch springform pan. Bake 8 to 10 minutes. Remove from oven; let cool.
2. In a small saucepan, heat cream on low heat until almost simmering. Whisk in chocolate, liqueur and orange zest until smooth. Let mixture cool slightly. Pour cooled mixture into graham-cracker crust. Gently tap bottom of pan on counter to settle mixture evenly. Cover and chill for 2 hours or overnight.
3. To serve, dollop marshmallow cream over top. (If using marshmallows, place on a nonstick cookie sheet and heat in oven until tops just begin to brown. Remove from oven and arrange as desired over tart.) Top with chocolate-covered graham crackers and sprinkle on orange peel. Makes 12 to 14 servings.