1. Cream together the butter and icing sugar very well until very smooth and creamy. No lumps of butter should be visible.
2. Blend in the vanilla extract and lemon zest.
3. Sift together the flour and corn starch.
4. Blend slowly into the creamed mixture along with the poppy seeds until a soft dough forms.Form into a 1 inch thick flat disk, wrap the dough in plastic wrap and chill for a couple of hours at least.
5. This dough will also last in the fridge for 3-4 days so that you can bake a few up at a time and enjoy them fresh from the oven if you like.To bake the cookies preheat the oven to 350 degrees F and line cookie sheets with parchment paper.
6. Roll the dough out to a little less than ¼ of an inch on a lightly floured surface. Use a 1 1/2 inch cookie cutter to cut out the cookies.
7. Place the cookies on the parchment paper a half inch apart. Bake for 12 - 15 minutes or until they just start to turn brown at the edges.Let cool for 10 minutes on the pan before transferring them to a wire rack to cool completely. Store in an airtight container.These cookies will freeze quite well for several weeks.

To make the Royal icing

1. Start by adding the icing sugar to a small bowl and add the lemon juice.
2. Add the egg white a tbsp at a time mixing until you get a very thick icing.
3. If the icing gets too thick add a little more egg whit e or lemon juice. f it gets too thin add more icing sugar until you get the right consistency.
4. Spread or pipe the frosting onto the cookies and let them sit out for several hours for the frosting to harden.
5. Store in airtight containers.