Chop the chocolate into very small pieces.
Place the chopped chocolate in a large heat proof bowl, and microwave for 45-second intervals on medium power (not high power, which is often the default). Stir the chocolate between each interval. Repeat the process until smooth.
In a very large bowl, beat the cream cheese and sugar until smooth. Turn off the mixer and scrape down the sides of the bowl as necessary.
Beat in the melted chocolate and vanilla extract.
Mix in the sour cream. Again, turn off the mixer and scrape down the sides and bottom of the bowl as necessary.
With the mixer on low speed, beat in the eggs 1 at a time until just combined. Stop mixing as soon as the eggs are mixed int.
Pour the batter over the baked crust and smooth the top.
Bake in the preheated oven for 30-35 minutes or until the top looks almost set in the middle and the cheesecake has a slight wobble.
Remove the cheesecake from the oven and gently trace around the outside of the cheesecake with a thin, sharp knife.