Preheat oven to 350F. Line an 8" x 8" baking pan with foil and lightly coat with cooking spray. Set aside.

Melt the butter in a small saucepan over low heat. Add sugar and stir until combined. Add chocolate chips, stirring until smooth. Remove from heat and add the eggs one at a time. Stir in vanilla until incorporated. 

In a small bowl, whisk together cocoa powder, cornstarch and salt. Add to chocolate mixture and use a hand mixer on high speed to beat the batter until smooth and shiny. Gently fold in the peppermint chunks. Pour into prepared baking pan.

Bake for 25 – 30 minutes until the brownies are set in the center. Remove and let cool. 

In the bowl of a standing mixer fitted with the paddle attachment, beat the butter on medium speed for one minute. Gradually add powdered sugar, mixing until combined. Add heavy cream, vanilla extract, peppermint extract and pinch of salt. Mix until light and fluffy, about 2 to 3 minutes. Scrape down the sides and bottom of the bowl with a rubber spatula if necessary. 

Spread over cooled brownies and sprinkle with crushed candy canes or sprinkles. Cut into squares to serve.