Line a slow cooker with 3 layers of baking paper, allowing the sides to overhang. Use an electric mixer to beat the butter, peanut butter and combined sugar in a bowl until pale and creamy. Add egg and vanilla. Beat to combine.
Add the flour and baking powder to the butter mixture. Stir with a wooden spoon until well combined. Stir in half the combined choc chips.
Press the mixture evenly over the base of the prepared slow cooker. Sprinkle over the remaining combined choc chips with the peanuts.
Cover and cook for 2 1/2-3 hours on low or until the edge of the cake is cooked through. Uncover and cook for a further 30 mins on low or until the cake is set. Turn the slow cooker off. Leave the cake in the slow cooker to cool slightly.
Use the paper to transfer the cookie cake to a serving board. Cut into slices. Serve warm with the ice cream and chocolate topping.