Preheat the oven to 175ºC/350ºF/gas mark 4

Put the Oreo cookies into a food processor and pulse until they have turned to fine crumbs

Put the butter into a microwave for 10-15 seconds to soften

Bake for 10 minutes then set aside to cool

Break the dark chocolate into pieces, putting them into a glass or pottery bowl

Put the bowl into a saucepan of water (just enough so that the bottom of the bowl doesn't come into contact with the water) and heat on a low flame until the chocolate has melted

While waiting for the chocolate to melt, put the cream cheese, icing sugar and cocoa powder into a mixing bowl and combine well

Break the eggs into a measuring jug, whisk lightly until just combined before adding little by little into the cheese/sugar/cocoa mixture, stirring after each addition

Add the melted dark chocolate to the mixture and combine well

Pour the mixture on to the biscuit base and bake in the oven for 60 minutes

Switch off the oven, leave the oven door slightly ajar and allow the cake to cool for half an hour

Put the cake on a wire rack until it has cooled completely

Break the white chocolate into pieces, putting them into a glass or pottery bowl with the double cream

Put the bowl into a saucepan of water (just enough so that the bottom of the bowl doesn't come into contact with the water) and heat on a low flame

When the chocolate has begun to melt, stir the mixture well and allow to cool until it has thickened but still pourable

Pour evenly over the top of the cake allowing some to drip down the edge

Serve at room temperature or refrigerated

Keep refrigerated and consume within 3 days