1. In large bowl, stir together flour and next 4 ingredients. In small bowl, whisk together peanut butter, milk, butter and vanilla. Stir into flour mixture and mix until smooth. Pour batter into 16-ounce microwave-safe mug.
2. Microwave on HIGH 1 minute and 30 seconds. Garnish with chocolate chips, chocolate syrup and melted peanut butter, if desired.