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.