In a food processor crush the Oreos into fine crumbs (you can also use a ziplock bag to do this step). Add melted butter to that and mix until combined.
Transfer the cookie crumbs into an 8-inch springform pan (nonstick/greased). Press the crumbs evenly onto the bottom of the pan, set aside.
Preheat oven to 325°f (160°c).
Cut the chocolate bar into small pieces unless you're using chocolate chips.Then melt the chocolate in a heatproof bowl on a double boiler, making sure the bottom of the bowl isn't touching the simmering water.
Once the chocolate is completely melted, set aside and let it cool slightly.
In a large bowl, beat the cream cheese and sugar on medium speed until smooth, scraping the sides of the bowl as necessary.
Add yogurt, whipping cream and vanilla essence, beat on low until combined. Add in the melted chocolate, cocoa powder and mix well. Then the eggs, one at a time, mixing on low speed after each addition just until mixed. Don’t over mix. Pour the filling over crust and spread evenly.
Using foil double wrap the bottom of the springform pan, sealing it tightly to prevent water from getting into the pan.
Place the springform pan into a larger pan/tray and pour boiling water into the larger pan, making sure the water level doesn’t come up higher than the foil. The water level shouldn't be more than a third of the springform pan.
Bake the cheesecake for 60-70 minutes or until the center of the cake is set but still slightly jiggly. The cake will set completely as it cools.
Allow the cheesecake to completely cool at room temperature. Then refrigerate overnight.
Carefully run a knife around the sides of the pan to loosen the cake, before releasing the cake from the pan and slicing. For a cleaner cut dip the knife into hot water before slicing.
Store the cake/cake slices in an airtight container in the refrigerator, for up to 5 days.