In a small bowl, beat the unsalted butter with the dark brown sugar and caster sugar until cappuccino-coloured, about two minutes, no more.
Beat in the beaten egg and vanilla extract.
Mix the flour with the bicarb and Maldon salt, then gently add that into the dough.
Finally, add the dark chocolate and roughly chopped pistachios.
Lightly (and I mean lightly) oil a 5" pan, then press the dough in, flattening and creating a slight depression in the middle, where you can add 1-2 tbsp of pistachio cream.
Bake at 180C or 160C fan for 14-15 minutes.
Leave to cool for 5-10 mins then eat.
