This is so good you'll want to make a second batch to try over brownie ice-cream sundaes!
Bring first 3 ingredients to a boil in a small saucepan over medium heat, stirring constantly. Cook, stirring constantly, 1 minute. Remove from heat, and stir in vanilla; cool slightly (about 10 minutes). Serve warm.
Make 3 days ahead; cover, chill, then reheat over low heat.