If I use my challah dough to make doughnuts and then eat lots of the fried doughnuts, what bracha would I make on those doughnuts?
The bracha on doughnuts is mezonos even if they were made using only water and flour. The reason is because they are fried. Food that is fried is considered as if it was cooked and therefore it will not be Hamotzei, even if you eat a ton of them (which anyways isn’t a good idea). There is an opinion that the bracha on doughnuts is hamotzei, and there are scrupulous people that will only eat them during a bread meal. However the minhag is to be lenient on this and they are eaten even not during a meal and the bracha on them is mezonos. Note: if the doughnuts are baked or fried in a pan with only a minimal amount of oil, such as when making a fried egg, then the bracha might be hamotzei, depending on the ingredients.
O:CH 168-13, M:B 69-70.