1. Bring first 3 ingredients to a rolling boil in a 3 1/2-qt. saucepan over medium heat, whisking constantly (about 7 minutes).
2. Stir in cream, and bring to a boil; remove from heat. Pour into bowl of a heavy-duty electric stand mixer. Gradually beat in powdered sugar and vanilla at medium speed, using whisk attachment; beat 8 to 12 minutes or until thickened. Use immediately.
Test Kitchen Tip: Don't panic if you overbeat the frosting. Thin to a spreadable consistency by adding 1 to 2 tsp. hot water.