Method

1. Position a rack in the centre of the oven and preheat to 350°F / 180°C.
2. Line a 9in / 22cm square baking pan with 2in / 5cm high sides with foil. Butter the foil.
3. Stir ¾ cup of the butter and the 6 oz / 170gms of unsweetened chocolate in a large bowl set over a medium saucepan of simmering water until melted and smooth. Cool until mixture is nearly room temperature.
4. Whisk in the sugar, vanilla, and salt. Whisk in the eggs 1 at a time.
5. Stir in the flour just until blended.
6. Stir in the 3 oz / 85gms of bittersweet chocolate and the walnuts.
7. Transfer the batter to the prepared pan.
8. Bake until a tester inserted into the centre comes out with moist crumbs attached, about 30 minutes. Cool the brownies on a rack.
9. Combine ½ cup of the cream and ½ cup of the mint in a small saucepan and bring to a simmer over medium-high heat, muddling and stirring often.
10. Place the finely chopped chocolate in a medium bowl. Set a strainer over the chocolate in the bowl. Pour the minted cream through the strainer and over the chocolate; discard the mint leaves.
11. Stir until the chocolate melts and the mixture is well blended. Cool to room temperature.
12. Whip the remaining 1 ½ cups of cream in a large bowl until soft peaks form.
13. Gradually fold the chocolate mixture into the whipped cream until well blended but still light and airy.
14. Using the foil as an aid, lift the brownie from the pan. Remove the foil.
15. Cut the brownies into squares.
16. Spoon the frosting generously atop the brownies.
17. Garnish with fresh mint and serve.