Combine brown sugar, cream and syrup (Jack optional) in a heavy sauce pan. Bring to a full, rolling boil and cook, stirring constantly, to the softball stage (234 degrees). Remove from heat and stir in butter and vanilla. Cool to lukewarm without stirring. Then add pecans (optional). Stir, stir & keep stirring until the fudge looses it's gloss. This will take some strong arming or you can try the kitchen aid stand mixer. When it is ready pour or plop into a pan lines in plastic wrap (this let's you drop it out after it is fully cooled). Cool for several hours.