Preheat the oven to 200°C, gas mark 6.
Line two baking sheets with non-stick baking parchment.
Put the almonds, icing sugar and cocoa into a multi mill attachment on the stand mixer; mill to make a well-mixed, fine powder.
Whisk the egg whites with a tiny pinch of salt using an electric hand whisk at full speed until stiff then fold in the almond mixture with a metal spoon.
Spoon into a piping bag and pipe circles onto the baking sheets. Bake, with the oven door left slightly ajar to allow steam to escape, for 8–9 minutes. Lift off the paper and cool on a wire rack.
To make the butter cream, put the sugar in a small saucepan with 50ml water; heat gently until dissolved.
Meanwhile, beat the egg yolks with an electric whisk until pale and creamy.
Boil the sugar syrup until it reaches the soft-ball stage (119°C on a sugar thermometer), which is when a little syrup forms a soft ball when dropped into cold water.
Immediately remove from the heat and pour onto the egg yolks, whisking until the mixture is thick. Cream the butter in a separate bowl, then beat in the cooled chocolate, egg mixture and pistachios.
Sandwich the butter cream between pairs of macaroons. Store between sheets of baking parchment; chill for 24 hours until they are chewy. Serve at room temperature