Preheat the oven to 180C/350F/gas 4. Line an 18 x 23cm or 20 x 20 square (7 x 9in, or 8 x 8 square) baking tin with parchment.
Mix the lemon and lime zest into the sugar. Stir the flours, flavoured sugar and salt together and pour in melted butter when it has cooled down. Mix well with a spoon.
Press the dough into the lined tin. You can mark lines with a sharp knife or a pastry cutter for cutting the fingers when baked.
Bake for 30-35 minutes until pale golden.
Remove from the oven and sprinkle with caster sugar (plain or mixed with more lemon zest). Cut the shortbread into fingers taken out of tin but still in the parchment– it will make 16 medium sized ones.
Cool them completely on a wire rack – they will be awfully crumbly at first but will set as they cool.
