I used extract in a hazelnut brown ale I brewed earlier this year. I basically looked at a couple clone recipes of Rogue Hazelnut Brown Nectar and at the info from Rogue ad formulated my own recipe based on that. The base beer had a roasty, nutty flavor to begin with so the hazelnut extract paired nicely with it. I bought my extract on ebay from a store called olivenation. I added 2 tsp for 2.5 gallons of beer. At first, the hazelnut flavor was a little strong, but after a month in the bottle it mellowed nicely and turned out to be pretty good and smells amazing. I've never added hazelnut liqueur but it sounds like an interesting idea.
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