Heat oven to 190C/fan 170C/gas 5. Lightly oil a baking sheet. Beat the butter, then cream it with the sugar and orange zest until light. Stir in the flour and baking powder and mix together to form a dough.
Divide the mixture into 10 balls, arrange on a baking sheet. Press each ball fl at with the tip of a round-bladed knife. Chill for 15 mins, then bake for 12-15 mins until light golden. Leave on the baking sheet for 2 mins before releasing with a palette knife. Slide onto a rack to cool. Will keep stored in an airtight container for up to a week.