Preheat oven to 350
Line baking sheet with parchment paper
Beat icing sugar, cocoa, cornstarch and salt on low to combine
Add egg white and beat on low until combined, then beat on high for 1 minute
Stir in vanilla and walnuts
Scoop tablespoon of batter on to prepared baking sheet
Bake until shiny and firm 12-15 minutes
Let cool on the baking sheet for 2 minutes before removing them to a cooking rack