Beat the sugars, butter and orange rind together in a large bowl with a hand-held electric whisk until smooth and pale. Add the beaten egg and vanilla extract and beat until combined.
Sift in the flour and baking powder and mix with a wooden spoon until all the ingredients are combined. Stir in the chocolate chunks and bring the mixture together with your hands to form a dough.
Transfer the cookie dough to a large sheet of clingfilm, roll the dough into a wide 7 cm (3 inch) sausage shape and wrap in the clingfilm, twisting the ends to seal. Chill in the refrigerator for 30 minutes.
Slice the dough into about 12 x 1.5 cm (¾ inch) thick discs and place, spaced apart, on 2 large nonstick baking sheets. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10–12 minutes or until golden brown around the edge and slightly paler in the centre.
Leave on the baking sheets for 2 minutes, then transfer to a wire rack to cool completely.