In a small saucepan, bring the maple syrup to a boil over high heat. Cook until it is reduced by half, about 5 minutes. Let cool slightly.
In a medium bowl, combine the syrup, butter, salt, and vanilla and mix until combined. Cover tightly and refrigerate for at least 1 hour, or up to 24 hours to allow the flavors to meld. Remove from the refrigerator 30 minutes before serving. The butt