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-qt. 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 until a toothpick inserted in the center comes out clean, 1-1/2-2 hours. Serve warm, with ice cream if desired.