1. Preheat oven to 350°.
2. Line an 8-inch square metal baking pan with aluminum foil, allowing foil to extend over edge of pan.
3. Place graham crackers in a food processor; process until finely ground. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Add flour and sugar to crumbs; pulse until combined. Add butter; pulse 15 times or until blended. Add egg; process until moist. Press crumb mixture into bottom of prepared pan.
4. Bake at 350° for 15 minutes. Cool completely in pan on a wire rack.
5. Cook cream in a heavy saucepan over medium-high heat to 180° or until tiny bubbles form around edge (do not boil). Remove from heat. Add chocolate, stirring until smooth. Fold in marshmallows. Spread chocolate mixture over cooled crust. Cover and chill 2 hours and 30 minutes or until firm. Holding edges of foil, lift mixture from pan, and carefully peel off foil. Place on a cutting board. Cut into bars.
Kids Can Help: My kids love pressing the crumb mixture into the pan, and then melting the chocolate and stirring in the marshmallows.