Arrange graham cracker pieces on bottom of lightly greased and waxed paper-lined 8-by-8-by-2-inch baking pan; set aside.
Melt chocolate in small saucepan over low heat. Remove from heat; stir in stir in 1 1/2 cups graham snacks, if desired, and marshmallows.
Spread mixture evenly over graham cracker-lined pan. Press remaining graham snacks on top of chocolate layer, if desired. Let stand until firm.
Cut into bars using bottom graham halves as guide. Store in airtight container.