Whizz the pistachios and 25g of the icing sugar in a food processor until fine. Mix with 175g more icing sugar in a large bowl. Whisk the egg whites until stiff, add the remaining 50g icing sugar and whisk again until thick and glossy. Fold into the nut mix. Spoon into a piping bag with a 1cm nozzle and pipe out 10p-coin-size blobs of mixture, well spaced. Poke any pointy tops down with a wet finger. If you don’t have a piping bag, just snip the end off a sandwich bag and use that instead.
Leave to dry for 30 mins, then bake for 12- 15 mins until risen. Cool on the paper. Beat the cream cheese with the chocolate and use to sandwich the macaroons together. Will keep for 1 day filled, or up to 3 days unfilled, stored in an airtight container.