Preheat oven to 400 degrees F (200 degrees C). Grease 2 tart tins with 1 teaspoon butter.
Mix graham crackers and 5 teaspoons butter together in a bowl until mixture is dense and moldable. Press graham cracker into the bottom of the prepared tins to form the crusts.
Combine chocolate chips and heavy cream in a pot over medium-low heat. Cook and stir until chocolate chips are melted, about 3 minutes. Pour mixture evenly over the crusts; top each tart with 4 marshmallows.
Bake in the preheated oven until marshmallows are golden brown, about 2 minutes.