Preheat your oven to 350°F (175°C). This is crucial for ensuring your brownies bake evenly.
Line your 9x13-inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a medium bowl, whisk together the flour, salt, and cocoa powder. Set aside. In a double boiler or microwave, melt the coarsely chopped dark chocolate and butter together until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the sugar and brown sugar. Stir in the melted chocolate mixture, then add the eggs one at a time, mixing well after each addition. Finally, stir in the vanilla extract.
Gradually add the flour mixture to the wet ingredients, mixing until just combined. Be careful not to overmix, as this can lead to tough brownies.
Pour the brownie batter into the prepared pan and spread it evenly. Bake for about 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan on a wire rack.
In a clean mixing bowl, whisk together the 2 tablespoons of flour with the milk and heavy cream until smooth. Set aside.
In another bowl, beat the softened butter and gradually add the sugar until light and fluffy. Mix in the creme de menthe and peppermint extract. Gradually add the milk mixture, beating until the filling is smooth and creamy.
Spread the mint layer evenly over the cooled brownie base. Chill in the fridge for about 30 minutes to set.
In a double boiler, melt the coarsely chopped dark chocolate with the unsalted butter and light corn syrup. Stir until smooth and glossy. Remove from heat and let it cool slightly before pouring it over the chilled mint layer.
Allow the ganache to set in the refrigerator for at least 1 hour. Once set, lift the bars out of the pan using the parchment overhang. Cut them into squares or bars with a sharp knife, wiping the knife clean between cuts for neat edges.
