Preheat your oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
In a large bowl, whisk together the melted butter and sugar until smooth and glossy, about 30 seconds.
Add the cocoa powder and whisk vigorously until no lumps remain and the mixture looks thick and fudgy.
Beat in the eggs one at a time, whisking well after each addition until the batter becomes lighter and slightly fluffy, about 1 minute total.
Stir in the peppermint extract and vanilla extract until evenly distributed.
Add the flour and salt, then fold gently with a rubber spatula just until no white streaks remain. Do not overmix or the brownies will be cakey.
Fold in 1 cup of the mint chocolate chips, reserving 1/4 cup for topping.
Pour the batter into the prepared pan and spread evenly. Sprinkle the remaining chocolate chips over the top.
Bake for 28-32 minutes, until the edges are set and a toothpick inserted 2 inches from the edge comes out with moist crumbs. The center should still jiggle slightly.
Cool completely in the pan on a wire rack for at least 2 hours before lifting out and cutting into 16 squares.
