Preheat the oven to 350 degrees F. Line a 13 × 9-inch baking pan with foil; spray lightly with cooking spray.
To make the crust, combine the graham cracker crumbs, sugar, and melted butter in a small bowl. Stir together with a fork until well combined. Press the mixture in an even layer into the bottom of the prepared baking pan. Bake 10 minutes; transfer to a wire rack while you prepare the filling. Reduce the oven temperature to 325 degrees F.
To make the filling, place the chopped chocolate in a heatproof bowl set over a pan of simmering water. Heat until melted and smooth, stirring occasionally. Remove from the heat and let cool slightly.
In the bowl of a stand mixer fitted with the paddle attachment, combine the cream cheese, sour cream and marshmallow cream. Beat on medium-high speed until smooth and well combined, 1-2 minutes. Blend in the cocoa powder, vanilla extract and eggs until incorporated. Stir in the melted chocolate until well blended. Pour the filling over the graham cracker crust and smooth the top with a spatula.
Bake about 25 - 30 minutes, or until the center is just set (does not jiggle when shaken gently.) Remove from the oven.
Evenly top the cheesecake layer with the mini marshmallows. Return to the oven for 2-3 minutes, just long enough so the marshmallows begin to melt into each other. Watch carefully! They brown very quickly.
Remove the pan from the oven and allow to cool to room temperature.
To make the drizzle design, melt chocolate chips carefully in the microwave at 50% for 20 to 30 second intervals, transfer the melted chocolate to a resealable sandwich bag with a tiny bit of the corner cut off, and make diagonal stripes each way on top of the bars.
Transfer to the refrigerator and chill 2-3 hours before slicing and serving.