Tip the dry ingredients into a large bowl. Heat the honey and butter in a pan over a low heat until the butter melts, then pour into the flour mixture along with the lemon zest. Mix well until the dough is combined and fairly solid. Cover and leave to cool.
Heat oven to 180C/fan160C/gas 4. Using your hands, roll dough into about 30 balls, each 3cm wide, then flatten each one slightly into a disk. Divide the biscuits between two baking trays lined with baking parchment, leaving room for them to expand. Bake for 15 mins, then cool on a wire rack.
To ice the biscuits, mix together the icing sugar, egg white and 1-2 tbsp water to form a smooth, runny icing. Dip the top of each biscuit in the icing and spread with the back of a knife. Leave to dry out in a warm