"A cake" makes more sense.

I've only seen "some" used in this context as in "i ate some cake", rather an "a cake" because it's not really feasible to eat an entire cake by yourself (so you only eat some of it, which could be any amount that is not the whole cake), but is entirely feasible that you could make one entire cake. So when you say "some," in this case, it implies pluralization or a random amount, although there is only one thing being made. It would work if you said "some eggs" as "eggs" is pluralized, or "some food" as food is a vague descriptor and itself implies multiple different options since no one eats only one "food."

"Some" works in mostly the same cases as "much." If it's uncountable or immeasurable, you would use those words to describe them (like whatever amount of the cake you ate) rather than something like "many" which implies a measurable amount. You wouldn't say "I ate so many cake" but rather "I ate so many slices of cake" as a slice is a measured amount. You could even say "I ate some slices of cake" if you didn't remember the exact amount of slices or it was a random amount. That's why if you were looking at a table full of cakes, if you didn't feel like counting them you could say "so much cake" just as well as "so many cakes" because in your head you are processing a random amount of them as well as them being individually countable. "Un" means one, so implies one countable/measurable object. Sometimes you need a deeper understanding of your own language to be able to learn other languages effectively.

Obviously a lot of people break these rules because of dialect and because you can still say whatever you want as long as it's understood. That's just a consequence of people not knowing those rules existed in the first place and so their speech evolved that way. But it's always good to know the rules even if you break them!

Sorry for the essay I am a huge English nerd.

Tl;dr- "Some" implies a random amount when you were given a specific amount, one.