Preheat oven to 200 degrees F. Line cookie sheets with parchment paper; set aside. In a large bowl, combine egg whites, vanilla, cream of tartar, and salt. Beat with an electric mixer on medium speed until soft peaks form (tips curl). Gradually add sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form (tips stand straight). Gently stir in lemon peel.

Transfer meringue mixture to a decorating bag fitted with a 1/2-inch round tip. Pipe 1-1/2-inch meringues 1 inch apart onto prepared cookie sheets.

Bake in the preheated oven for 1-1/2 to 1-3/4 hours or until meringues appear dry and are firm when lightly touched. Do not open the oven during baking time. Transfer cookies to a wire rack; let cool.

Just before serving, spread about 1 teaspoon Lemon Frosting on the bottoms of half of the cookies. Top with remaining cookies, flat sides down. Filled meringues can sit at room temperature for 2 hours before serving. Makes about 36 sandwich cookies.

In a medium bowl, combine 1/3 cup softened butter and 2 teaspoons finely shredded lemon peel. Beat with an electric mixer on medium to high speed until fluffy. Gradually add 2 cups powdered sugar, beating until blended. Add 1 tablespoon lemon juice, 1 teaspoon at a time, until frosting reaches desired consistency. Makes about 1 cup.

Layer unfilled cookies between sheets of waxed paper in an airtight container; cover. Store at room temperature for up to 2 days or freeze for up to 1 month.