Preheat oven to 350ºF. Grease and line a small rectangular baking dish.
Base: In a mixing bowl, combine butter, flours and cornstarch to form a dough. Add sugar and chopped peanuts. Add a little water if needed to bring together. Press into the pan. Bake for 25 minutes, then let cool.
Peanut layer: In a medium skillet, add butter and peanut butter and cook, stirring, over medium heat until melted and combined. Add confectioner's sugar and mix together. Pour over shortbread and chill in fridge for 2 hours.
Pour caramel over the peanut layer, then chill for 1 ½ hours.
Melt the dark chocolate, then pour over the caramel layer. Chill for 2 hours, then add decorations and enjoy!