In a small saucepan, combine raspberry vinegar and shallots. Cook over medium-high heat until vinegar is reduced to 2 tablespoons. Add cream and continue cooking until liquid is reduced to 2 tablespoons. Remove from heat; whisk in 2 or 3 pieces of butter, one piece at a time. Place over low heat and cook, whisking in remaining butter [one piece at a time] until mixture is almost the consistency of mayonnaise. Whisk in jam. (Be careful not to over-cook, or you'll break the emulsion.)