Put the butter and water into a pot, and stir to melt. Don't worry about them separating, they are not suppose to mix. Bring it to a boil, remove from heat, dump in all the flour at once and stir like mad until it becomes a yellow batter. Return to stove and continue stirring the batter, until it shrinks into a somewhat cohesive ball. Don't worry if its not actually all together. As long as the batter is no longer sticky and the dough leaves the side of the pot you are ok.
Now comes the workout. Let the dough cool just a little so you don't get scrambled eggs when you add the eggs. The eggs have to be added slowly, 3 tablespoon at a time. Make sure you use large eggs, not the tiny ones. Eggs are instrumental in the rising, and this is one point not to screw around with. Equally important is the beating in. Don't be lazy and add it all in. So, if making a full batch, beat in 3 tablespoon of egg with an electric whisk each time. Otherwise, just beat it in with a wooden spoon. This pastry is more stubborn than usual when it comes to taking in the egg, but keep at it and it'll turn smooth in no time. Once all the eggs have been beaten in, the batter is done, and should look like this.
Pre-heat oven to a hot 210 degree celcius. The baking is a tad troublesome as well, but the satisfaction of a puffed, tall and golden puff is quite worth the while for me, though I must say by itself the pastry taste like crap. I happen to be going through custard mania now, which is why I chose to make this. Huge glop of sweet custard in the empty puff, delish. So, most recipes would say to sprinkle water on the tray and inside the oven to create steam to help with the puff's rising. I personally think it a waste of time, but I suppose it does help a little. I also find it weird to put batter onto a wet tray, which is why I don't do it.
Another important thing to note is NOT to open the oven door until the puffs are fully risen and has baked for around 10 - 15 minutes at least. Premature opening of the doors leads to a drop in temperature and pressure within the oven, which leads to flat puffs. Once the puffs are fully risen and browned, pry them off the foil and poke a hole at the bottom, then return to oven to dry out some more. One way of testing its done-ness is to tap it; it should sound hollow. Remove puffs after around 5 minutes, place on rack to air out and pray that it stays upright. Here's my final [standing!] product:
And next comes the custard. Bring milk to a boil, and while it is heating, whisk together the egg yolk, sugar and flour in a cooking pot. Once the milk boils [no need for it to boil vigorously, just as soon as bubbles start to appear], pour into egg mixture, whisking quickly. Return to stove, and KEEP STIRRING. Do not leave the custard one step, and I mean it. They burn faster than you can blink. Although, if it is burned only a little, don't despair, taste it and see if its obvious. Usually the custard can take a little burning before it becomes inedible, so if you don't mind little black specks in your custard you can still get away with it. Make sure to scrap every inch of the pot as you stir, as it is easy to miss a spot, and that untouched spot can quickly become a charred mess.
As soon as the custard thicken enough to coat your spoon or fork, take it off the heat. If left on too long custard becomes lumpy, but make sure to taste it before you stop cooking. You want to make sure all the floury taste has been cooked out of the batter, or it will taste, well, like flour, which is gross. Let the custard cool JUST A LITTLE before laying a piece of cling film right over the surface of the custard. As the custard cools the surface hardens into a layer of skin, which will make for lumpy custard if not removed, which is why you can't wait til the custard cools to put on the cling film. What the cling film does is make it easy to remove the hardened skin; it basically peels off together with the film. Chill in the fridge and your custard is ready to go.
Also, 60g of cocoa powder is actually quite a lot, as they are rather intense in flavour. Mixing them up first, with all the other ingredients except the chocolate, will allow you to gauge how much more chocolate to add. Why waste good chocolate in an already sinful chocolate sauce? Conversely, if you can't get cocoa powder, just omit it and substitute with more chocolate, how much up to your budget and your taste buds. While I love dark chocolate passionately and can't understand milk chocolate lovers, I do know there are many out there. I have to confess this is a seriously deep and dark sauce, so if you are inclined towards milk chocolate, simply reduce the amount of water and substitute with milk, and use milk couverture instead of dark ones. At my insistance my sister spent a lot of time trying to capture the lusciousness of this sauce, and so I'll post a few more of them here.
*Recipe adapted from Food and Wine*
All that's left now is the assembly! Its dead easy and takes like 5 minutes. Remove the custard and discard the cling film. Take 1 puff and turn it over on the serving plate. If the hole is not big enough widen it. The smaller puffs would require a piping bag, but this huge puff comes with a huge hole, so just spoon it in.
And finally, these beautiful puffs are ready to be served!