Preheat oven to 180C / 160C fan / Gas mark 4. Grease 2 baking trays.

Cream butter and sugar together until light and fluffy

Beat in the egg yolk, then add the flour, lemon zest and chocolate chips and mix in.

Add the milk gradually until the dough comes together.

Turn the dough onto a floured surface and knead it gently into a ball, then use a floured rolling pin to flatten the dough out to about 1/2 cm.

Use a round cutter (or shape of your choice, we used 6cm diameter) to cut out the biscuits and put on the baking trays with a small gap between.

Bake for 8 minutes. Whisk the egg whites then remove the biscuits and brush on egg white then sprinkle with some additional caster sugar before baking again for about 6 minutes until they're light golden brown.

Remove from the oven, leave to cool for 5 minutes before putting the cookies onto a wire cooling rack to cool fully.