Line a shallow 20cm (8 inch) square baking tray with baking paper.
To make the first layer: in a large bowl, mix all the ingredients together except the butter.
Pour the melted butter over and combine well. Using a spoon, press this into the tin and chill in the fridge for 15 minutes to set.
To make the second layer: cream the butter with the custard powder, ideally with an electric hand whisk, until fluffy. Then add the icing sugar a little at a time.
Add a little warm water if it gets too stiff. Smooth the creamy mixture on top of the biscuit base, and again place in the fridge for 15 minutes to set.
To make the third layer: melt the chocolate and butter in a heatproof bowl over a saucepan of simmering water until smooth and glossy, making sure the base of the bowl does not touch the water.
Let it cool slightly, then pour over the custard layer and smooth with a palette knife. Chill for a further 15 minutes, or until set.
Take out of the fridge 30 minutes before serving to allow it to soften slightly, then cut into bars with a large sharp knife.