1. Combine whipping cream and next 4 ingredients in a small saucepan. Carefully scrape seeds from vanilla bean into saucepan. Add vanilla bean to saucepan, and cook mixture over medium heat, whisking constantly, until smooth (about 2 minutes). Reduce heat to medium-low. Cook, whisking constantly, 2 to 3 minutes or until thickened.
2. Remove saucepan from heat. Carefully remove vanilla bean. Stir in butter until melted. Serve immediately. Store sauce in an airtight container in refrigerator up to 5 days.
Note: To reheat, warm sauce in a small saucepan over low heat, stirring in 1 to 2 tsp. whipping cream as needed to thin sauce.