These can be eaten as a snack, as a finger-food meal (with raw vegetables or a salad) or as the potato part of a meal. We like them so much that even if we have them as part of a meal, we make a double batch to have some left over to snack on. They're good hot or cold, even the next day, but don't reheat well.
I had to look up a recipe for choux pastry, because I never measure ingredients any more: I know what the right amount looks like in my pan! But of course your pan is bound to be different from mine.
Put water, butter and salt in a spacious pan and heat until it's boiling and the butter is dissolved. Turn the heat down and add all the flour at once. Stir vigorously until the mixture is smooth and leaves the sides of the pan (you may think this is never going to happen, but eventually it will). Remove from heat, add one egg and mix it in thoroughly, then the other egg, and stir until it's glossy. This is the choux pastry, which you can also make to use for something else like eclairs.
Mix in the mashed potato, for instance with the kneading attachment of an electric beater. At this stage you could already make plain potato choux, but they're better when flavoured.
Preheat the oven to 200 degrees C. Line a baking sheet with baking paper.
Add something highly flavoured to the dough: for instance 50 g grated sharp cheese, or the drained contents of a tin of anchovies chopped up small, or a few tablespoonfuls of pesto; anything that goes with potatoes. The dough is so bland that you have lots of scope for experiment.
Put a dessertspoonful of the dough at a time on the baking sheet. The most convenient way to do this is with two spoons, one to scoop up the dough and one to push it onto the paper. Leave about a centimeter between the blobs because they will rise. You can put the dough in a pastry bag and pipe it into nifty shapes, but I don't usually do that because it's a lot of work already and they'll taste just as good.
Bake about 15 minutes on high heat, then another 15-20 minutes at 175 degrees (your oven may take longer than mine), until the choux are golden brown and smell delicious.