Preheat the oven to 350ºF. Line a 9x13-inch baking dish with a parchment paper sling.
In a large bowl, whisk or beat the melted butter, sugar, eggs, vanilla, and peppermint extracts until combined.
1 cup salted butter, 2 cups (400g) granulated sugar, 4 large eggs
Stir in flour and cocoa powder by hand using a spatula or wooden spoon. Gently stir in pecans or chocolate chips. Pour the brownie batter into the prepared pan.
1 1/2 cups (212g) all-purpose flour, 1/2 cup unsweetened cocoa powder, 1 cup chopped pecans, 2 teaspoons pure vanilla extract, 1 teaspoon peppermint extract
Bake for 30 to 35 minutes until a toothpick inserted into the center comes out with just a few crumbs clinging to it. Let the brownies cool completely.
While the brownies cool, make the frosting by beating softened butter, heavy cream, vanilla extract, peppermint extract, and powdered sugar in a large bowl until creamy and light, about 3-4 minutes. Stir in the crushed candy canes toward the end.
2 cups powdered sugar, 1/2 cup salted butter, 4 Tablespoons heavy cream, 1 teaspoon vanilla extract, 1/2 teaspoon peppermint extract, 1/2 cup crushed candy canes
Frost the cooled brownies with the peppermint frosting and sprinkle with some additional crushed candy cane pieces on top. Cut into squares and serve.