Beat the butter and sugar in the bowl of an electric mixture until pale and creamy. Gradually add the eggs, beating well after each addition, then add the vanilla. Add the flour, baking powder and chocolate and beat until combined.
Spoon half the mixture into a lightly greased 12x8 baking pan lined with non-stick baking paper. Top with spoonfuls of cream cheese (I just sort of tore off bites of it). Sprinkle with the oreo cookies. Top with the rest of the dough and spread it to cover the cookies and cheese.
Bake for 25-30 minutes or until it is cooked though when tested with a skewer. Allow to cool in the tin before slicing.
To melt white chocolate in a microwave - using 50% power, microwave for 30 seconds at a time. Stirring between times. Stop when most of the chips or chunks are melted and keep stirring, the rest will melt.