Heat oven to 180C/160C fan/gas 4. Line a 20cm square cake tin with baking parchment. Whizz 125g of the pistachios, 75g of the chocolate and about half the sugar in a food processor until very finely chopped. Tip into a bowl with remaining sugar, butter, eggs, flour and milk, plus a pinch of salt, and beat. Scrape into the tin and bake for 35-45 mins until a skewer poked in comes out clean. Cool.
Melt the remaining milk chocolate, stir in the soured cream and cool until spreadable. Trim the edges off the cake, then split in half. Spread a little icing over the base, sandwich the top of sponge back on. Spread remaining icing over the top, roughly chop the remaining pistachios and scatter them over. Cut into 16 squares to serve.