Preheat the oven to 325°F and line a cookie sheet with parchment paper.
Cream the butter and sugar in a bowl and, when you have a light, soft, whipped mixture, beat in 1/3 cup cocoa powder (sifting if it is lumpy) and, when that's mixed in, beat in the flour with the baking soda and baking powder. Or just put everything in the processor and blitz, if you prefer.
This mixture is very soft and sticky and I find it easiest to form the cookies wearing my disposable vinyl gloves, so pinch off pieces about 1 tablespoon in size, and roll them into balls, then lightly flatten into fat discs as you place them, well spread, on your cookie sheet; you should get about 12 on at a time.
Bake each batch for 15 minutes; even though the cookies won't feel as if they've had enough time, they will continue to cook as they cool. They will look slightly cracked on top, and it's this cosy, homespun look I love.
Remove the cookie sheets to a cold surface and let it sit for 15 minutes before transferring the cookies to a wire rack, with a sheet of newspaper under it (to catch the drips while topping them).