1. Preheat oven to 350°F. Line a 9x9x2-inch baking pan with foil; set aside. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar, lemon peel, baking soda, cream of tartar, and salt. Beat until combined, scraping sides of bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour.

2. Pat half of the dough into the prepared pan. Bake for 12 minutes or until browned. Use foil to lift cookie from pan and cool on a wire rack. Cool pan. Repeat with remaining dough.

3. Soften ice cream by placing it in a very large bowl and stirring until creamy. Add lemon curd; fold gently to swirl. Peel foil away from cookie squares. Line the 9x9x2-inch pan with plastic wrap. Place one baked cookie square in the pan. Spread with ice cream mixture. Top with remaining cookie square. Cover and freeze at least 4 hours or until firm. Cut into squares. Roll sides in crushed candy, if desired. Wrap individually and freeze to store.

Lemon Curd

1. Finely shred peel from 2 lemons and juice all of the lemons. In a small saucepan stir together the sugar and cornstarch; stir in lemon peel and juice. Cook and stir over medium heat until thickened and bubbly. Stir half of the mixture into the egg yolks. Return mixture to saucepan. Cook and stir until just bubbly. Cook and stir 2 minutes more. Remove from heat. Strain through a fine mesh sieve into a small bowl. Gradually add butter pieces, stirring after each addition until melted. Cover surface with plastic wrap and chill at least 45 minutes.