Preheat the oven to 180ºC, 350ºF, Gas Mark 5.
Gently melt together the butter, sugar and molasses (or maple syrup if using) until the sugar is dissolved. Stir in the remaining ingredients and mix really well. Spoon into an 18cm square tin and smooth over the top.
Bake for 20 minutes until golden. Leave to cool in the tin, marking the portions with a sharp knife but not cutting through until completely cold.