Preheat oven to 350°F. Grease and line two 8-inch pans.
Whisk flour, graham crumbs, sugar, baking powder, soda, and salt.
Whisk milk, sour cream, melted butter, eggs, and vanilla. Fold into dry mix until just combined.
Divide batter and bake 25–30 minutes. Cool completely.
Make ganache: heat cream to simmer, pour over chocolate, stir smooth. Chill until spreadable.
For icing: beat butter, add marshmallow fluff and vanilla until light and fluffy.
Layer cake: spread ganache between layers, crumb-coat with marshmallow icing, chill 15 minutes.
Finish with a final coat of icing. Torch mini marshmallows carefully or brown briefly under broiler, then sprinkle crushed graham crackers.