Stir together the dry ingredients in a large bowl, then add eggs, milk and melted butter. Mix well to form a smooth batter.
Place an oreo at the bottom of all 12 muffin holes (make sure the tray is greased first), then spoon a little of the mixture over the top, to just cover the oreo. Add a square of white chocolate on top of this, then spoon some more mixture over the top. Careful not to add too much as the mixture rises during baking.
Bake for 20 minutes then leave to cool in the tray on a wire rack. When cool enough to handle, turn the tray over and serve.