Grind the oreo cookies in a food processor or blender or crush them in a ziplock bag with a rolling pin. In a bowl, combine the cookie crumbs with melted butter. Mix well.
Divide the crumb mixture between 6 (4-oz) Mason jars, press it with the back of a spoon.
If using the baking chocolate bar chop it roughly or add in chocolate chips, place in a bowl, microwave for 20-second intervals, stirring after each interval until the chocolate has melted. Stop when most of the chocolate has melted and few pieces remain, they will melt on stirring. Stir well until all the chocolate has melted and is smooth and glossy. You may even melt the chocolate in a bowl using the double boiler method. Make sure the bowl is not touching the water or else the chocolate will seize. Set the melted chocolate aside to cool slightly.
Next, in the mixing bowl of your stand mixer beat the cream cheese and sugar on low speed until smooth. You may use a hand blender or just a whisk too.
Add the melted chocolate, continue beating low medium-low speed.
Add the egg and beat on medium-low speed until well incorporated.
Add the sour cream and the vanilla extract and beat well to mix.
Pour about 3 tbsp of the cheesecake filling in the prepared oreo crust in each jar. Give it a gentle tap on your palms to eliminate any bubbles. Cover the jars with the lid that has the rubber ring, then close the jar using the outer band, but do not tighten the lid all the way, just enough to cover the jar in order to prevent any condensation from dripping in it.
Add 1.5 cups of water into the inner steel pot of the Instant Pot. Place a trivet and stack the jars offset over the trivet.
Close the lid of the Instant Pot, and set the valve to 'SEALING'. Pressure cook on 'HIGH' for 6 minutes. It will take some time for the pressure to build up after which the timer will start.
Let the pot depressurize naturally (about 15 minutes), once the silver pin drops, turn the knob to 'VENTING' and open the lid. Transfer the jars to a wire rack with the help of tongs or gloves. Let the cheesecake cool completely before pouring the chocolate ganache.
For the chocolate ganache: Chop baking chocolate bar roughly, place in a bowl, add the heavy cream, microwave for 20-second intervals, stirring after each interval until the chocolate has melted. Stop when most of the chocolate has melted and few pieces remain, they will melt on stirring. Overheating will turn the chocolate into a thick, lumpy, grainy mass and looks dull. Divide the ganache between 6 jars, cover an refrigerate until ready to serve.
Let the chocolate cheesecake jars stand at room temperature for 10 minutes before serving. Serve these chocolate cheesecake jars with some whipped cream, and chocolate shavings. Scoop up some and enjoy!