S'more Sundae

For hot fudge: In a 1-quart heavy saucepan, combine cream, corn syrup, and salt. Cook over medium heat until just simmering, stirring occasionally. Remove from heat. Add chocolate (do not stir); let stand 5 minutes. Stir until smooth. Stir in vanilla. Set aside to cool slightly. (Fudge can be prepared 10 days ahead and refrigerated. It will get very firm; just microwave about 15 seconds to soften.)

For marshmallow creme: Place egg whites and cream of tartar in the bowl of a stand mixer fitted with a whisk attachment. Beat to soft peaks on medium-high speed, about 3 minutes. Stop mixer.

In a small saucepan, combine sugar, corn syrup, the water and salt. Cook over medium heat, stirring gently. When sugar has begun to dissolve, increase heat to high and clip on a candy thermometer. Boil until mixture reaches 240°F. Remove from heat.

With the mixer running on low speed, slowly drizzle about 2 tablespoons hot sugar mixture into egg whites to warm them. Continue drizzling, until all the hot sugar mixture has been added. Increase speed to medium-high and beat until marshmallow is stiff and glossy, 7 to 9 minutes. Toward the end of the beating time, beat in vanilla. (Marshmallow creme can be made up to 2 weeks ahead; store in an airtight container in refrigerator.)

For caramelized graham crackers: Preheat oven to 350°. Line a 15x10x1-inch baking pan with foil. Arrange graham crackers snugly in pan, overlapping slightly and breaking as necessary to completely cover bottom of pan.

In a small saucepan, combine butter, brown sugar, salt and cinnamon. Bring to a boil over medium-high heat, stirring frequently. Boil gently, uncovered, 2 minutes without stirring. Remove from heat and stir in vanilla.

Pour hot butter mixture over crackers and, working quickly, spread with a spatula to evenly cover them. Bake until bubbly, 8 to 10 minutes, rotating pan halfway through baking time. Cool completely on a wire rack before breaking into small pieces.

To assemble sundaes: Scoop chocolate and vanilla ice cream into bowls. Top with warm hot fudge, then marshmallow cream. Sprinkle with caramelized graham cracker pieces.