Put 3 marshmallows in a microwave-safe bowl and microwave until melted, about 30 seconds. Mix with 1/2 cup ricotta and season with salt. Chill 1 hour. Meanwhile, toss 2 sweet potatoes (peeled and cut into 1/2-inch cubes) with 1 tablespoon olive oil, 2 teaspoons chopped rosemary and 1 teaspoon salt on a baking sheet. Bake at 425˚ until tender, about 30 minutes. Thinly slice 1 baguette and toast in the oven. Top with the ricotta mixture, strips of prosciutto or speck and the sweet potatoes. Season with pepper.