Heat the oven to 180°C/160°C Fan/350°F, and get out a - preferably light-coloured - baking sheet. You don't need to line it if it's non-stick; otherwise, lie a sheet of baking parchment on it.Stir the flour, cocoa, baking powder, bicarbonate of soda and fine sea salt together in a small bowl just to combine them.In a slightly larger bowl - I use a pudding basin that I now can't look at without thinking of these cookies - vigorously beat the butter, both the sugars and the vanilla with a small wooden spoon until you have a buff-coloured and creamy mixture. If you aren't a messy person, you could use a cereal bowl for this.Add a generous spoonful of the dry ingredients to the creamed butter and sugar and beat it in gently with your wooden spoon. Then - still gently, unless you want cocoa and flour all over the place - beat in the rest of your dry ingredients, in about three batches. Once the dry ingredients are absorbed, you can beat vigorously until you have a sticky, rich-brown dough, that clumps together, at which point you can stir in the chocolate chips.It's not often I demand this level of precision, but I now weigh this mixture, and divide it in two; you don't need to be fanatical about this, a few grams here or there won't make the difference. Squidge each half in your hands to form two patties about 7cm / 2½ inches in diameter and place them on your baking sheet, at least 10cm / 4 inches apart, as they spread while cooking.Sprinkle ⅛ teaspoon of sea salt flakes over each cookie, and bake in the oven for about 12 minutes, until the top of each biscuit is riven with cracks. At 10 minutes - which is when I start checking - they will be utterly smooth, but in the next 2 minutes they seem to transform themselves. I crouch by the oven, staring through the cloudy glass door feeling like, as the old Joan Rivers joke has it (and forgive me if you've heard me tell this before), Elizabeth Taylor shouting 'Hurry!' at the microwave.Once the surface is cracked, and the cookies have spread, they are ready. They will, however, feel very soft - even uncooked - to the touch, and you will doubt me. But I will forgive you, as long as you obey me. So whip out the baking sheet, leaving the cookies in place for 5 minutes. Only then may you slip a metal spatula under the cookies and tenderly transfer them to a wire rack. For optimal eating pleasure, leave for another 10 minutes before biting into one. I often succumb after 5, which is perfectly permissible, I feel, though I should warn you that the biscuit is unlikely to hold its shape by then. But in times of urgent need, such matters of form scarcely matter.