In a bowl, combine 1/4 cup sugar, flour and baking powder. In another bowl, combine the milk, peanut butter, oil and vanilla; stir into dry ingredients just until combined. Spread evenly into a slow cooker coated with nonstick cooking spray.
In a bowl, combine the 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 of cake comes out clean. Serve warm with ice cream.