Lightly coat the inside of a 3-1/2- or 4-quart slow cooker with cooking spray; set aside.
In a medium bowl stir together flour, 1/3 cup sugar, 2 tablespoons cocoa powder, and baking powder. Add chocolate milk, oil, and vanilla; stir just until moistened. Stir in peanut butter pieces, semisweet chocolate pieces, and peanuts. Spread batter evenly in the prepared cooker.
In another medium bowl combine 3/4 cup sugar and 2 tablespoons cocoa powder. Gradually stir in the boiling water. Carefully pour cocoa mixture over batter in cooker.
Cover and cook on high-heat setting for 2 to 2-1/2 hours or until a toothpick inserted into center of cake comes out clean.
Remove liner from cooker, if possible, or turn off cooker. Let stand, uncovered, for 30 to 40 minutes to cool slightly. To serve, spoon pudding cake into dessert dishes. If desired, top with ice cream and chocolate bar pieces.