Orange Marmalade Sauce

Bring orange juice to a boil in a medium saucepan. Reduce heat to medium and cook, uncovered, until reduced by half, 20 to 30 minutes. Add marmalade and brown sugar; return to a simmer, whisking until smooth. Mix cornstarch with water in a small bowl; add to sauce and cook, whisking, until slightly thickened, about 1 minute. Remove from heat and stir in butter and vanilla . Let cool slightly; serve warm.