- In a bowl, cream the butter and sugar for 2 minutes with an electric mixer.
- Add the egg and lemon zest. Beat for 1 minute.
- In another bowl, combine the flour, baking powder and salt.
- With a wooden spoon, add the dry ingredients to the butter mixture and beat well until smooth.
- Roll the dough into a 7.5-cm (3-inch) log. Cover in plastic wrap and refrigerate for 3 hours.
- With the rack in the middle position, preheat the oven to 180 °C (350 °F).
- Unwrap the roll of dough and place on a work surface. Cut into 1/2-cm (1/4-inch) thick slices and place them on a baking sheet lined with parchment paper.
- Bake for 15 minutes or until cookies begin to brown.
- In a bowl, blend the icing sugar and egg whites. Add a little water if necessary to get a smooth glaze that will spread evenly. Set aside about 125ml (1/2 cup) of icing for decorating.
- Frost cookies, let dry for a few minutes then add some yellow coloring to the reserved frosting, to mimic the color of an egg yolk.
- With a spoon, add a little icing in the centre of each cookie to give it the appearance of a sunny side up egg.