Stir the flour, 1/2 cup sugar, and baking powder in a large bowl. Add milk, oil, and vanilla and stir until smooth. Stir in the peanut butter.
Spray a 6 or 7-quart slow cooker with cooking spray. Spread the batter in the bottom of the crockpot. Press Rolos evenly across the top of the batter.
Whisk 1/2 cup remaining sugar and cocoa powder in a medium bowl. Slowly whisk in the boiling water. Carefully and slowly pour the chocolate water over the top of the cake. Do not stir.
Cover and cook on High for about 2 hours, or until a toothpick comes out clean. Cool slightly before serving. Serve with ice cream or whipped cream.
Note: all slow cookers cook differently. My crockpot has a “High 6 hour” setting, which is what I used. Be sure to check on your cake after 1 1/2 hours and it may take up to 3 hours, depending on how your machine cooks.