Preheat oven to 325°.
Pulse graham crackers in a food processor until finely ground. Add butter and pulse until crumbs begin to clump together. Press crumbs firmly into an even layer in a foil-lined 8x8 pan.
Bake crust at 325° for 10 minutes.
While crust is baking, bring cream and milk to a simmer in a large saucepan. Remove from heat and whisk in chocolate, vanilla, and salt until smooth.
Warm eggs under hot running water to bring them to room temp, then whisk into chocolate one at a time. Pour chocolate mixture over baked crust.
Bake at 325° for 25-30 minutes, until chocolate is just set in the center.
Scatter marshmallows over the top and gently press into the chocolate filling.
Return to the oven and broil on high until marshmallows are golden brown.
Let bars cool and refrigerate for at least 30 minutes to let filling completely set before cutting.