1. Whisk together first 5 ingredients in a small saucepan. Cook over medium heat, whisking constantly, 3 minutes or until sugar is dissolved. Bring to a boil. Reduce heat to low, and cook, whisking constantly, 3 to 4 minutes or until mixture is thickened. Remove from heat, and whisk in peanut butter and vanilla. Serve immediately.
Note: To make ahead, prepare recipe as directed. Cover and chill up to 24 hours. Warm sauce in a small saucepan over low heat, stirring in 1 to 2 teaspoons milk as needed to thin sauce.