1. Preheat the oven to 180C/350°F.2. In a mixer, combine the butter and the 2 sugars until the mixture is fluffy. Add the egg and mix.3. Add the flour, salt, and baking powder, and mix quickly. Add the zest and chocolate chips, and mix gently.4. Form 2cm diameter balls of dough. Refrigerate the dough for 10 minutes.5. Slice the dough and place on a baking sheet lined with parchment paper. Space the biscuits well apart.6. Bake for 6 minutes.7. Remove from the oven and let cool on a rack before serving.