Preheat oven to 180C / 350F /160C fan forced. Line 2 large baking trays with baking paper.
Sift together the flour, cornflour, cocoa and baking soda.
In a separate bowl, beat together the butter and both sugars until lightened and fluffy.
Scrape down the sides of the bowl and add half the flour mix. Beat until just combined. Add the milk and remainder of the flour and mix until it just starts to come together.
Finally, add the pistachios and give the mixture another beat until mixed through and the dough is clumping
Pull the dough together with your hands and then roll into a long log about 1 1/2 inches thick. Wrap in plastic wrap tightly and place in the fridge for 1/2 hour (or in the freezer for later).
Cut the log into 36 pieces Bake for around 10 minutes, turning the trays halfway through.
Allow them to cool on the trays for a few minutes before using a spatula to help to transfer them to a cooling rack. Once cool, they will firm up and be crispy on the outside and just slightly chewy in the very centre.
Once cool, drizzle with extra melted chocolate and sprinkle with pistachio crumbs