1. Preheat oven to 350F. Line a baking sheet with parchment paper.
2. In the bowl of a stand mixer, or using a handheld mixer, mix together butter and sugar until well combined. Stir in eggs until combined. Stir in flour, cornstarch, baking powder and salt just until combined.
3. Chill dough in refrigerator for 10 minutes.
4. Place dough on lightly floured piece of parchment paper. Fold parchment paper over dough, then roll dough to ¼ inch thickness. Cut into desired shape. Cut out center of cookies with small round cookie cutter. Bake on prepared baking sheet for 10 minutes. Allow to fully cool.
5. Sugar Glass: In a microwave safe bowl, mix together corn syrup and sugar. Microwave for 30 seconds, then stir. Microwave an additional 2-3 minutes, stirring every 30 seconds, or until temperature reaches 300F. Pour into the center of 2/3 of the cookies. Allow to harden 10 minutes.
6. To assemble: Spread icing around outer edge of cookie, then place cookie with no sugar glass on top. Place mini M&M’s in center, then spread icing around outer edge of cookie and place a cookie with sugar glass on top. Decorate with additional icing if desired.
