In a large bowl, combine the bread flour, sugar, and salt. Add the active sourdough starter and water, mixing until a shaggy dough forms. Knead the dough for about 10 minutes until smooth and elastic. Cover the dough and let it ferment at room temperature for 4-6 hours or until doubled. Gently fold in the chocolate chips, marshmallows, and crushed graham crackers into the dough. Shape the dough into a loaf and place it into a greased loaf pan. Let the dough proof for another 1-2 hours until it has risen visibly. Preheat the oven to 220°C (425°F). Spread the unsalted butter on top of the loaf for a golden crust. Bake for 35-40 minutes or until the crust is deep golden brown and the loaf sounds hollow when tapped. Allow the bread to cool completely on a wire rack before slicing and serving.