Simmer the cream in a small saucepan. Remove from the heat. Add the chocolate and whisk until smooth. Set aside.
Spray a baking sheet with nonstick cooking spray. Place 6 of the Maria crackers on the baking sheet and top each with half a marshmallow. Bake until the marshmallow starts to brown, about 5 minutes. Carefully arrange the crackers on a serving dish.
Top each with 1 tablespoon chocolate ganache and another Maria cracker. Serve warm or at room temperature.