To make combine butter and sugar in a large bowl with an electric mixer on high speed until light and fluffy. Add eggs and vanilla extract and beat on medium speed until blended.
Gradually add in flour, cornstarch, and salt and mix on low speed until well combined. Place the dough on plastic wrap, flatten it to a disc, and wrap the dough in the plastic wrap. Refrigerate the dough for 30 minutes.
Roll out half of the dough on a lightly floured surface to 1/4-inch thickness. Using a circle cookie cutter, cut out the cookies. Place on a parchment-covered baking sheet. Repeat with the remaining dough until you have cut out all of your cookies.
Bake for 10 to 12 minutes or until the edges of the cookies are lightly browned at 350 degrees F. Remove and cool completely on a wire rack.
In a small bowl combine butter, Hawaiian Shaved Ice Flavored Syrup, and powdered sugar. If the frosting is too thin add more powdered sugar and if the frosting is too thick add a little milk. Tip: If you are going to make multiple flavors of frosting as I did you will want to divide the butter and powdered sugar in half for each flavor.
Once the cookies are cooled, frost the bottom of one cookie and place another cookie on top. Add more frosting to the edge of the cookie and roll in the flavor of your choice of Cotton Candy Express Sugars. Tip: Place your frosting in a sandwich bag and cut the corner to make a piping bag to frost your cookies.
I made the following flavor combination for my Easter Sandwich Cookies: Blue Hawaiian Frosting rolled in Pink Vanilla Express Sugars, Cotton Candy Frosting rolled in Orange Cotton Candy Express Sugars, and Cotton Candy Frosting rolled in Blue Raspberry Express Sugars. Have fun making your favorite flavor combinations.