Grease a square baking tin with butter and set aside. The bigger the tin, the thinner the bars will be.
Add the cereal to a large mixing bowl. Set to one side.
In a small saucepan over a medium low heat, add beanut butter and maple syrup. Stir until well incorporated, then, stirring occasionally, heat until bubbling. Then set a timer for two minutes and stir frequently during this time. Remove from the heat; the mixture will resemble thick caramel.
Carefully pour over the dry cereal and stir and fold until well-incorporated.
Pour into the baking pan and press down using a rubber spatula to form a smooth even layer. Leave to set at room temp or in the fridge for about half an hour.
Turn out and place on a chopping board.
Weigh out the chocoalte and peanut butter; microwave in short bursts and stir until combined. Pour the chocolate mixture over the bars and spread into an even layer. Leave to set then cut into individual bars.