Preheat oven to 180°c and line a 20x20 brownie tin with baking parchment.
Add the rolled oats to a high powered food processor and blend until a fine flour forms. Add all the remaining base ingredients and blend again until thoroughly combined. Spoon into the prepared tin and firmly press the mixture down. Bake for 10 minutes until slightly browned. Place to one side whilst making the caramel.
Clean the food processor then add all the caramel ingredients and blend until a thick, smooth caramel forms. This will take 5-10 minutes of blending, scraping down the sides as required.
Spread the caramel evenly over the base and place in the freezer to set for 30 minutes.
Break dark chocolate into small pieces and place in a heatproof bowl with the cashew butter and microwave in small increments until, melted and smooth. Finally spread the chocolate layer over and place in the fridge for 1 hour to set.