Start by whisking together flour, sugar, salt, and crushed graham cracker. Cut in chilled butter until it looks like peas in the flour. Add ice water gradually until the dough comes together, form a disk, and chill.
Heat cream in short intervals, then pour over chopped chocolate. Stir until smooth and creamy. Chill in the fridge until ready to assemble.
Roll the dough to ⅛ inch thick and cut into six rectangles. Chill them briefly so they hold shape while filling. This prevents any leaks when baking.
Pipe chocolate ganache on three rectangles and add marshmallows. Top with the remaining rectangles, press edges gently, crimp with a fork, and brush with egg wash.
Bake at 350º F for 30–35 minutes until golden. Mix powdered sugar, cocoa, meringue powder, milk, and vanilla for icing. Pipe or drizzle over cooled tarts for that classic chocolate finish.