Both are good. The difference is that one uses the pronoun "what" whereas the other uses an adverb "how". "What" represents the thing itself (the cake). "How" represents the method (how it was baked). So it depends whether it is the THING or the METHOD that you did not like.

Both sentences are grammatically correct, but they have slightly different meanings. "The cake isn't what I wanted" means that the cake did not meet your expectations. This could be because it did not taste the way you wanted it to, or it did not look the way you wanted it to. "The cake didn't turn out how I wanted" means that the cake did not turn out as you had planned. This could be because you made a mistake while baking it, or because the ingredients you used were not what you expected. In the context of your sentence, "The cake isn't what I wanted" is probably the better choice. This is because you are simply stating that you did not like the cake, without specifying why. If you wanted to be more specific, you could say "The cake was too dry" or "The cake didn't have enough flavor." However, if you wanted to explain why you did not like the cake, then "The cake didn't turn out how I wanted" would be a better choice. This would allow you to explain what went wrong with the cake, and why it did not meet your expectations. Ultimately, the best choice of sentence will depend on the specific context in which you are using it.

They are all fine sentences, slightly different meanings: 'I just made a cake, but I didn't like it.' (You could also say: 'but it wasn't very good/tasty.' And 'The cake didn't turn out how I wanted.' These two sentences are about you making a cake and then not liking the result; in the first, this is explicit (i.e. you say explicitly that you made the cake). In the second sentence, it's implied that you made it and that it didn't turn out like you wanted. 'Turn out' is used correctly and is often used to describe the final product when cooking; this sentence sounds very natural. 'The cake isn't what I wanted.' This doesn't include any direct or implied information about if you made the cake yourself. It's a more general statement about you getting a cake and then not liking it.