Line a 15 x 11 x 1-inch jelly-roll pan with nonstick aluminum foil, or line pan with regular foil and lightly coat with nonstick cooking spray.
Gently melt chocolate in large bowl set over saucepan of gently simmering water; stir chocolate until smooth. Remove bowl from over water and let cool slightly, about 3 minutes.
Stir the marshmallows, peanuts and graham crackers into the melted chocolate. Spread the mixture evenly into the prepared jelly-roll pan.
Place the pan in the refrigerator for 1 to 2 hours or until the bark is completely cooled.
Invert the bark onto a large cutting surface. Peel off the aluminum foil. Cut the bark into thirty-six 2-inch pieces.
Store the bark in an airtight container at cool room temperature for up to 2 weeks.