Grease an 20cm/8 inch square cake pan and line with a strip of baking parchment, making sure that it overhangs either side so that it is easy to lift the baked brownies out of the pan. Preheat the oven to 190°C/375°F/gas mark 5.
Roughly chop half of the nuts, not too small, and set aside. Place the butter and chocolate in a pan over a low heat and stir until melted and smooth. Set aside.
Place the eggs and sugars in a large bowl and beat with an electric mixer until extremely thick and pale and the sugar has almost dissolved.
Beat in the chocolate and butter mixture followed by the bourbon and vanilla extract.
Sift together the flour, cocoa powder and salt then fold it into the chocolate mixture until no pockets of flour remain. Stir in the chopped nuts.