Preheat the oven to 350 degrees F (175 degrees C).
Mix crushed graham crackers, melted butter, and brown sugar in a bowl until well combined. Set aside 1/4 cup graham mixture for topping; press the remainder into a 9x13-inch pan.
Bake in the preheated oven for 10 minutes. Set aside to cool.
Dissolve raspberry gelatin in boiling water. Add frozen raspberries, stirring until melted. Chill until partially set, then spread over graham crust.
Pour milk into a microwave-safe bowl and heat in the microwave until warm. Stir in marshmallows until melted; allow to cool slightly.
Fold whipped cream into marshmallow mixture, then spread on top of raspberry mixture. Sprinkle with reserved graham topping. Chill in the refrigerator for 3 to 4 hours before serving.
