I guess the problem is that "It was delicious" suggests that the cake is gone/finished/has been eaten. If that is the case, then it is impossible to say "Would you like some?" because it appears there is none left.
If you say "I made a cake this morning. It is delicious." OR "I made a delicious cake this morning" then there is no suggestion that it has all been eaten, and therefore it is perfectly reasonable that you are offering someone a piece of the cake.

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