https://m.youtube.com/watch?v=dQw4w9WgXcQ
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It depends on the temperature and type of both donut and filling. Too warm or too sweet is icky for a custard donut. Custard is good if you don't mind something that's a bit heavy and not super sugary. If you make them too sweet, then it's just far too rich and sweet and dense. Jam, on the other hand, is good for light and sweet, because of the cocktail of other flavours that go into a jam. Cinnamon is similar. Glazed is good alone but not if you've already eaten. Of course, this shifts around depending on your palate's tolerance for fats and sugars.
In conclusion, custard donuts serve a purpose and you can't have them for ither purposes. That will be all, thank you for listening to my thesis.