Learn Italian: Questo cioccolato sa di tartufo. - This chocolate tastes like truffle.

While sapere usually means "to know" a fact, when it is paired with the preposition di, it becomes a sensory verb meaning "to taste of" or "to smell of". Since chocolate is a food, it naturally translates to "tastes like" in this context.

Could I use come instead of di, since the English translation is "tastes like"?

No, that would sound unnatural in Italian. To express that something has the flavor or scent of something else, the verb sapere specifically requires the preposition di. If you used come, you would need a completely different noun-based construction like ha un sapore come il tartufo, but sa di is the standard, most idiomatic way to say it.

Since sapere di can also mean "to smell of", how do you know which sense is meant here?

You rely entirely on context! If you are eating the chocolate, sa di tartufo means it tastes like truffle. If you are simply holding it up to your nose, the exact same sentence means it smells like truffle.

Why is there no article before tartufo? Shouldn't it be del tartufo or un tartufo?

When using sapere di to specify a flavor or scent, the noun that follows acts more like an adjective describing the quality of the taste. Because you are talking about the general concept of a truffle flavor rather than referring to one specific physical truffle, the article is dropped.