In a bowl, over a saucepan of simmering water, melt the chocolate; stirring occasionally; set aside.
Using a stand mixer with the beater attachment or handheld beater, beat the cream cheese and sugar together until smooth and creamy; scrape down the sides of the bowl as required.
Add the vanilla; then the eggs, one at a time, beating until each is well incorporated.
Add the sour cream and salt; beat well and fold through the melted chocolate.
Pour the filling into the cookie base and smooth over the top as much as possible.
Place the pan on the middle shelf of the oven.
Place in the pre-heated oven and bake for around 45 - 50 minutes or until the center is almost set; there should be a very slight wobble in the center.
Remove from the oven and allow to cool, at room temperature, for at least 45 minutes before transferring to the refrigerator to cool completely.