Cream the butter and castor sugar together until combined and fluffy. Add the egg, vanilla extract and lemon zest and mix in.Add the flour and baking powder and mix. Use your hands to bring everything together into a dough. Shape the dough into a disc, wrap in plastic and refrigerate for at least 1 hour.Preheat your oven to 180°C. Line a baking tray with baking paper. Roll the dough out to a thickness of 3-4mm on a lightly floured surface. Cut out your shapes and arrange them on the tray. Bake the cookies for 10-12 minutes or until golden around the edges. Transfer to a cooling rack.To make royal icing: whisk the egg white for a few seconds until foamy. Sift in half of the icing sugar and use an electric beater to mix until combined. Sift in the remaining icing sugar. Beat the mixture for 3-4 minutes until thick and glossy. Add a drop of two of gel food colouring, mix and spoon into a piping bag.To make glacé icing: sift the icing sugar into a bowl. Add a small amount of water and mix. Continue to add a little bit of water at a time until you have a thick icing. It should be runny enough to spread but thick enough to coat the back of a spoon. Add a drop or two of gel food colouring and mix well.Once the biscuits are completely cool decorate with the icings and sprinkles. Allow to dry before serving. Reprinted with the permission of Cupcakes & Couscous. To see more click HERE.