Grease a shallow 28 cm x 18 cm 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 half 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.