Welcome to the Coffee Forum. You'll find a lot of information here.
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Regarding your question about a hazelnut latte.....(assuming you're making it at home) the easiest way would be to add a shot of hazelnut syrup to your coffee before adding the steamed/foamed milk. I've seen people make the latte and then pour the shot of hazelnut syrup straight through the middle it. Then, they serve it without stirring it.