Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan.
Mix the melted butter and sugar in a bowl, then whisk in the eggs and vanilla.
Combine the dry ingredients (cocoa powder, flour, salt, baking powder) in another bowl, and fold them into the wet ingredients until just combined.
Pour the batter into the pan and bake for 20-25 minutes. Let cool completely before frosting.
For the frosting, beat softened butter until fluffy, then gradually add powdered sugar, peppermint extract, and heavy cream. Spread or pipe the frosting on the cooled brownies.
Garnish with crushed candy canes and optional white chocolate curls.