Heat the oven to 400°F.
Unfold the pastry sheet on a lightly floured surface. Roll the pastry sheet into an 18x9-inch rectangle. Cut into 18 (3-inch) squares. Press the pastry squares into 18 (2 1/2-inch) muffin-pan cups. Spoon 1 teaspoon peanut butter into each pastry cup. Top each with 1 square chocolate and about 3 marshmallows.
Bake for 10 minutes or until the pastries are golden brown. Let the pastries cool in the pan on a wire rack for 10 minutes.