Equipment: two baking sheets lined with silicon baking sheets; a sturdy ziplock plastic bag; a pair of scissors.
Preheat the oven to 420°/ F.
Cut up the butter, and put it with the water and salt in a pan. Heat over medium heat until the butter has melted. Dump in the flour, and mix vigorously with a wooden spoon, until the flour is completely incorporated. The dough ball should slightly film the pan with a floury residue. Take off the heat and cool the bottom by putting it on a damp kitchen towel, or briefly running cold water over it.
Add the eggs, little by little, to the pan, mixing vigorously between additions. It may look like someone threw up at first, but it will all get incorporated.
Scoop the warm dough into the plastic bag. Cut the end off one corner with scissors, to make an opening about .5 cm / 1/4 inch or so. Now you will squeeze the mixture onto the pans - the shape dictates what you'll call it eventually. You can make long blobs, which would make them eclairs, tiny blobs, or bigger blobs. To smooth out any parts that stick up when you squeeze out the dough, just wet your finger in water or milk and smooth out. You can also glaze the puffs with a beaten egg wash, to give a shine to it. In this case use a pastry brush, and smooth out the top with it as you glaze.
Baking time also varies, depending on how big your blobs are. For about 5 cm diameter / 2 inch puffs, bake for about 20 minutes, rotating the pans halfway through. (Rotating means to switch the pans so that the top one is on the bottom, and vice versa. You may also want to turn the pans at this point too.) Watch the puffs and when they are a medium brown color, take them out.
When the puffs are done, immediately slash into each of them with a serrated knife. This lets out the steam and prevents the inside from getting soggy. You can then return the puffs to the oven for a few minutes. To ensure there will be no sogginess whatsoever, some people cut open the puffs and let them dry out a bit in the oven, but I sort of like some softness inside so omit this step.
You can also poach the dough in boiling water, to make light gnocchi. Squeeze blobs of the dough into the salted boiling water and cook until the bits float to the surface. Serve with butter, cheese, fresh parsley and so on.