Grease a shallow 11" x 7" baking pan with butter; line bottom with parchment or waxed paper. Coarsely grind fruit and nut mix in a food processor or blender. Melt chocolate and butter together in a small bowl placed over a pan of gently simmering water, stirring frequently. Mix in the processed nut mix and 1/2 of the orange zest. Spread mixture smoothly in prepared pan. Sprinkle remaining zest over top, pressing it in lightly. Refrigerate 1 to 2 hours or until firmly set. Loosen set mixture from pan by carefully running the tip of a knife around the inside edges of the pan. Carefully turn out onto a board; remove paper. Turn mixture over and cut in 32 pieces. Store in an airtight container in a cool place for 2 to 3 days.