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.
Long story short, Beer.