LINE 13x9-inch pan with foil or parchment paper, with ends of foil extending over sides of pan. Arrange graham wafers on bottom of pan, cutting as needed to completely cover bottom of pan; set aside.
MICROWAVE semi-sweet chocolate and peanut butter in medium microwaveable bowl on MEDIUM 1-1/2 to 2 min. or until chocolate is completely melted when stirred, stirring every 30 sec. Stir until well blended. Pour over grahams; spread with spatula to cover completely.
REPEAT microwave melting steps with the white chocolate. Drop spoonfuls of the white chocolate over chocolate-covered grahams. Immediately cut through the chocolate mixtures with knife several times for tiger-stripe effect.
REFRIGERATE at least 1 hour or until firm. Use foil handles to remove dessert from pan. Peel off foil. Cut dessert into bars. Store in tightly covered container in refrigerator.
serving size = 1 bar (27 g)