Put the chocolate in a small heatproof bowl. In a small saucepan, heat the cream until hot to the touch. Pour the cream over the chocolate and let stand until melted. Whisk until smooth.
Preheat a griddle or large skillet. Using a serrated knife, trim off the top and bottom of the pound cake so that the cake is about 1 1/4 inches thick. Carefully split the pound cake in half horizontally. Spread the peanut butter on one half and the marshmallow on the other, leaving a 1/2-inch border all around. Sandwich the two halves together and spread the top and bottom with the butter. Place on the griddle and cook over high heat, turning once, until golden and nearly warmed through, about 2 minutes. Transfer to a work surface and cut the cake into twelve 3/4-inch-wide strips. Serve at once with the chocolate sauce.