Line a slice tin with baking paper. Put shortbread biscuits in a food processor and pulse until coarse. Add 6 tablespoons of butter and combine. Using your hands, press mixture into base of prepared tin. Freeze for 5 minutes to firm.
Meanwhile, combine chocolate morsels, cream, lemon zest and remaining 2 tablespoons of butter in a small saucepan over a low heat and cook, whisking often, until melted. Reserve 2 tablespoons of the roughly chopped pistachios, then stir remainder into chocolate mixture. Set aside to cool for a couple of minutes.
This recipe was adapted from Better Homes Gardens Australia magazine