In a large bowl, beat the milk, peanut butter, oil and vanilla until well blended. In a small bowl, combine 1/4 cup sugar, flour and baking powder; gradually beat into milk mixture until blended. Spread into a 1-1/2-quart slow cooker coated with cooking spray.
In a small bowl, combine cocoa and remaining sugar; stir in boiling water. Pour into slow cooker (do not stir).
Cover and cook on high for 1-1/2 to 2 hours or until a toothpick inserted near the center comes out clean. Serve warm with ice cream.