Preheat the oven to 200°C, fan 180°C, gas 6. Melt the butter in a large pan over a medium heat. Remove the pan from the heat and stir in all the remaining ingredients.
Line an 18cm x 28cm baking tray with baking parchment and press the mixture into it. Bake for 25 mins.
Remove from the oven and, using a knife, score out 18 squares on the surface. Cool in the tray before cutting into pieces. Will keep for up to 10 days in an airtight container.