Melt the chocolate over a pan of boiling water (or in the microwave, if you prefer), and leave to cool. Separately, sift together the flour, cocoa, baking powder and salt.
Cream the butter with the brown sugar until it's pale and fluffy, then add the eggs, vanilla, and melted chocolate; to this, add the flour mixture and milk.
Divide the resulting dough into four batches, and wrap each of them in clingfilm. If you're finding this difficult, you can just place each lot of dough into a separate bowl and cover. Allow to rest in the fridge for 30 minutes.
Preheat your oven to 180°C (gas mark 4), and divide each batch of dough into 16 (or less, if you'd rather have larger cookies).
Roll each piece of dough into a ball, then roll in the granulated sugar, coating the entire ball; do the same with the icing sugar, allowing it to coat the entire piece of dough.
Place the dough balls onto baking trays (I always line mine with greaseproof paper, but if you have the silicon/non-stick variety, you can probably skip this step); bake for about 15 minutes, then allow to cool. I'd advise placing a baking tray or kitchen towel under your cooling tray to catch excess icing sugar. And, that's it - you're done!