This recipe is based on the original version that came from a friend; this is what I actually did when making it.

Preheat oven to 180°C. Melt butter in a saucepan, crush biscuits finely, mix the biscuit crumbs into the butter, and press into a cake tin. Place the cake tin in the fridge. Beat cheese and sugar together until smooth (I used a whisk); add the eggs and vanilla essence and continue beating until you've got something that looks like pancake batter. Melt the chocolate. Pour the mixture into the cake tin, drop the chocolate in and use a knife to swirl the chocolate into the filling (it'll harden rapidly, so don't hang around). Bake for 30 minutes or until knife comes out clean when inserted in centre of cheesecake. Allow to cool, then refridgerate overnight.