Preheat oven to 180 C / Gas 4. Lightly grease one 20x30cm baking tin.
Mix together the flour and the caster sugar.
In a saucepan, combine the Dr Pepper, cocoa and 225g butter; bring to the boil. Pour melted butter mixture over the flour and mix well. Stir in the beaten eggs, bicarbonate of soda, vanilla, buttermilk and mini marshmallows; mix well. Pour batter into the prepared tin.
Bake at 180 C / Gas 4 for 30 to 35 minutes.
Meanwhile make the peanut butter topping. Cream together the remaining 90g butter, dark brown soft sugar and the peanut butter. Add the milk and stir well. Mix in the nuts.
Remove cake from oven and spread peanut butter topping over warm cake. Place cake under the grill, about 10cm from the heat source. Grill for just a few seconds or until topping starts to bubble. Do not scorch. Let cake cool at least 30 minutes before serving.
If you can't find Dr Pepper, Coca Cola works a treat, too.