Preheat oven to 300F (150C). Place oven rack at lowest level. In a large mixing bowl, using a wooden spoon, beat butter with icing sugar, orange peel and salt until well blended. Stir flour into butter mixture in 4 batches. Stir in chocolate chips, if using, with last flour addition. Using floured fingers, pat mixture into 2 ungreased (9-inch/23-cm) pie plates or (8-inch/2-L) square cake pans or one of each size, evenly pressing down edges. Using a fork, pierce dough every inch (2.5 cm). Bake in 300F (150C) oven until shortbread edges are lightly browned and centre is golden, from 35 to 40 min. Sprinkle top with granulated sugar, if you like. Cool completely on a rack before cutting into squares or bars. Store in a tightly sealed container in the refrigerator for up to 3 weeks.