Preheat oven to 350°F (177°C). Line a 9x13-inch baking pan with parchment paper and set aside.
Melt 1 cup butter and 8 ounces semi-sweet chocolate together in a medium saucepan over medium heat, stirring constantly until smooth. Remove from heat and whisk in the granulated sugar. Allow to cool slightly.
Whisk in the eggs, one at a time, then whisk in the vanilla extract.
Gently fold in the flour and salt until just combined. Spread batter evenly into prepared pan.
Bake for 30–35 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs. Cool completely in the pan.
For the mint layer: Using a handheld or stand mixer, beat 1/2 cup softened butter on medium speed until smooth. Add confectioners’ sugar, heavy cream, peppermint extract, and green food coloring. Beat until creamy and spread evenly over cooled brownies. Refrigerate for 1 hour.
For the chocolate topping: In a small saucepan over low heat, melt 6 ounces chocolate and 1/4 cup heavy cream, stirring until smooth. Pour over the mint layer and spread evenly. Chill brownies for another hour until topping is set.
Lift brownies out of the pan using parchment overhang. Slice and serve chilled or at room temperature.