Pulse flour, cocoa, sugar, salt and baking soda in the food processor several times till their mixed well. Cut the butter into 10-12 cubes and add them to the flour mixture. Pulse several times till the mixture looks like coarse sand. Combine the milk and vanilla in a small cup. With the processor running, add the milk mixture and continue to pulse until the mixture clumps around the blade or the sides of the bowl. Transfer the dough to a large bowl and knead a few times to make sure it is evenly blended. Form the dough into a log about 2 inches in diameter. Wrap the log in cling film or foil and refrigerate for at least one hour or till firm enough to slice neatly.
If you’re making the dough by hand:
Sieve flour, cocoa, sugar, salt and baking soda in a bowl. Mix well with a fork. Rub the cubes of butter into the flour mix with your fingers, as you would while making pie dough, till the mixture resembles coarse sand. Add the milk tablespoon by tablespoon till the mixture just come together. Like the recipe states, I needed about 5-6 tablespoons of milk, but you might require less. Do not overwork the dough. Gather into a log, wrap and chill as mentioned above.
Preheat the oven to 180°C. Line baking sheets with parchment paper. Cut the log of dough into slices about 1/4 inch thick and place them one inch apart on the lined sheets. Bake for 12-15 minutes. The cookies will puff up a little and deflate and they’ll be done 1 to 1 1/2 minutes after they deflate. Cool the cookies on racks. The cookies turn crisp on cooling completely. If they still remain a little spongy in the middle they haven’t been baked long enough. Pop them into the oven for a couple more minutes and then cool again. Grab a book and sink into the bed with a handful of them.