Whip up the cream cheese, sugar and egg yolk together until smooth and fluffy and set aside.
Preheat the oven to 350F/170C and line a rectangular baking tray.
Place a bowl over a pan of simmering water and add the chocolate and butter.
Leave to melt, stirring occasionally.
Remove from the heat and stir in the sugars to dissolve.
Sift in the flour and leave for a few minutes to cool.
Add the eggs and then pour into the tray.
Dollop blobs of the cheesecake mixture on top and swirl around.
Bake for 35-40 minutes.
They'll still wobble when they come out of the oven so leave to cool in the tin.
Slice and serve up!