Combine the flour, cocoa powder and baking powder in a bowl. Place the butter and chocolate in a medium saucepan over medium heat. Cook, stirring, for 2-3 mins or until melted and smooth. Remove from heat. Stir in the sugar until combined. Add the egg and stir until combined. Add the flour mixture and stir until just combined. Fold in peanut butter and peanuts until lightly marbled. Spoon into prepared slow cooker and smooth the surface. Cook, covered, on low for 3 hours.