1.Spray oval 5-quart slow cooker with cooking spray.
2.In large microwavable bowl, mix brown sugar, butter, pumpkin pie spice, salt and water. Microwave uncovered on High 1 1/2 minutes, stirring once. Stir in vanilla. Add sweet potatoes; stir to coat thoroughly.
3.Pour mixture into slow cooker. Cover; cook on High heat setting 3 1/2 to 4 hours or until fork tender. Reduce to Warm heat setting until ready to serve.
4.Stir mixture; top with marshmallows, and cover 1 to 2 minutes or until marshmallows melt slightly