There are two schools of thought on filling jelly doughnuts. Purists (often Austrians, who enjoy small and delicate krapfen during waltz season) frequently claim that the only proper way to fill a jelly doughnut is before frying. How does one do that? The answer: by cutting a thin round of dough, laying a spoonful of jam on top of it, laying another dough round over the top and sealing it as one would a ravioli (raviolo?). You then carry on with the proofing and frying as per usual. This avoids the low-brow hole at the side of the pastry, which only serves to advertise that fact the maker has been poorly brought up.
I’ve tried this method and it’s a major pain in the Fastnachtsküchelchen. Not least because it requires you to roll the stretchy dough out to a mere 1/8 inch, which makes the evenness of the sheet very hard to judge. Unless the rounds are perfectly uniform, the jam will shift to one side of the pastry or the other during proofing. Why is that a big deal? Because an uneven load of jam will weight down the doughnut like a buoy as it fries, making it nearly impossible to flip over. Submerging the doughnut with your tongs or spider is one way to go, though that means you can only fry one doughnut at a time, plus you miss out on the traditional pale stripe around the doughnut’s beltline. And woe to those whose doughnuts burst in the oil…a gooey oil-degrading mess.
Using a syringe tip is a far easier method, even though it reveals the flaws in my upbringing. But this is a doughnut we’re talking about here, am I right? And anyway I haven’t waltzed in decades.