Line a loaf tin with baking paper, leaving a little overhanging the edges.
Process the dates and nut butter in a food processor for a few minutes, until the mixture is well combined and forms a paste. Transfer to a large bowl, and stir through the puffed rice and chopped nuts.
Press the mixture firmly into the lined tin (it will be quite sticky so you may want to use wet hands to do this). Pour the melted chocolate over the top of the slice, and place in the fridge to set for 1 hour. Cut the slice into squares and serve.
Keep the slice in the fridge until ready to eat as it will soften at room temperature.