Grease a 9 x 13 inch (23 x 33cm) baking pan and line the base and sides with parchment paper. Preheat the oven to 350F (180C).
Stir the butter and 11 oz (300g) chocolate in a large saucepan over medium heat until both are almost melted. Remove from the heat & continue to stir until mixture is smooth.
Add the sugar, eggs & zest stir until well combined.
Add flour, baking powder, cocoa & salt into the chocolate mixture; stir until well combined.
Stir in the extra chocolate & pistachios; spread into the prepared pan. Bake, on the centre rack, in the preheated oven for 35 minutes or until just firm. (It will set more as it cools.) Leave to cool in the pan. Cover & place in the fridge until serving. Cut into about 45 pieces to serve.