Heat oven to 350 degrees F. Line a 8" x 8" baking pan with foil and spritz with cooking spray.

For the brownies, melt the 1/2 cup butter in a large saucepan over low heat. Remove pan from heat and stir in the 1/2 cup cocoa powder until smooth. Stir in the sugar, followed by the eggs, stirring after each addition. Stir in the 1 teaspoon vanilla extract, 1/2 teaspoon peppermint extract and salt. Add the flour and stir until fully incorporated.

Pour batter into the prepared pan and bake 25 minutes. Cool completely.

For the frosting, combine the 3 tablespoons softened butter with 3 tablespoons cocoa powder, corn syrup, 1/2 teaspoon vanilla extract and 1/4 teaspoon peppermint extract. Beat until smooth. Add the confectioners' sugar and 1 teaspoon milk. Add more milk by the teaspoon as needed to achieve spreading consistency.

Frost cooled brownies and sprinkle with crushed peppermint candies.