I tried making hazelnut coffee today by grinding up hazelnuts and adding them to the coffee grounds for the vacuum-pot brew.
It worked, slightly. Very little flavor transferred, but the flavor that did make it into the coffee was the hazelnuts’ bitterness (like eating them directly), not the syrupy-sweet flavor that people usually mean when they want “hazelnut-flavored” anything.
It was an interesting experiment. While it didn’t make the coffee bad, I won’t try it again.