Prep: Preheat the oven to 350° F and line an 8x8 baking sheet with parchment paper and then set it aside. Crush the peppermint candy in a ziplocked bag using a rolling pin.
Dough: In a small bowl whisk to combine the flour, baking powder, and cornstarch. Then set this dry mixture aside.
In a separate bowl, using an electric mixer, combine the butter, sugar, egg, and peppermint extract. Beat until creamy.
Add the dry ingredients to the butter mixture and mix again until all ingredients are fully combined. Add the crushed peppermint candies to the bowl, reserving about ¼ cup for topping the blondies later, and stir to combine.
Bake: Transfer the dough into the prepared baking pan and smooth out the top with a rubber spatula, pressing it out evenly into all sides and corners of the pan.
Place it in the preheated oven and bake for 35 minutes, or until the top of the blondies is golden brown and a toothpick inserted in the center comes out with moist crumbs. Allow the blondies to cool completely before frosting.
Frosting: In a bowl of an electric mixer, combine the powdered sugar and butter, mix until fully combined. Then add 3 TBSP of milk and beat until the frosting is smooth and creamy. Add the remaining tablespoon of milk only if the frosting is too dry.
Proceed to spread the frosting evenly on the cooled blondies. Top them with the remaining crushed peppermint candy pieces, slice into 9 equal squares and serve. Enjoy!