Hello, Yaniv, and welcome to the Grammar Exchange!
Yaniv Kimelfeld posted:Can I say
I ate some of the cake.
Yes, you can.
Yaniv Kimelfeld posted:Is 'cake' countable or uncountable in this case?
It's countable. You could even say that you ate a bite of each of the cakes. If you said, "I ate some cake," then "cake" would be uncountable.
Yaniv Kimelfeld posted:What about purely countable nouns like nectarine?
You can say: "I ate some of the nectarine."