Preheat your oven to 325°F (160°C). Line an 8×8-inch square baking pan with parchment paper, leaving an overhang on the sides so you can easily lift the bars out once they are set.
In a medium bowl, combine the chocolate graham cracker crumbs and the melted butter. Stir until the crumbs are evenly moistened. Press the mixture firmly into the bottom of your prepared pan. Bake for 8 to 10 minutes, then remove and let it cool while you prepare the filling.
In a large bowl, beat the softened cream cheese and sugar together until the mixture is smooth and free of lumps. Add the eggs one at a time, beating on low speed after each addition. Stir in the sour cream and vanilla extract until just combined. Be careful not to overbeat the batter, as this can cause cracks.
If using raspberry jam, warm it slightly in the microwave for 10 seconds to make it easier to pipe. If desired, add a drop of red food coloring to make it look more like “blood.” Transfer the puree into a piping bag with a very small round tip or a plastic sandwich bag with a tiny corner snipped off.
Pour the cheesecake batter over the cooled crust and smooth the top with a spatula. To create the brain look, pipe the raspberry puree in a series of tight, squiggly lines across the top. To make it look like a brain, pipe two distinct sections (hemispheres) with a small gap running down the center.
Bake the bars for 35 to 40 minutes. The edges should be set, but the center should still have a slight jiggle. Turn off the oven, crack the door open, and let the cheesecake cool inside for about 30 minutes to prevent cracking.
Remove the pan from the oven and let it cool completely at room temperature. Once cool, refrigerate the bars for at least 4 hours, or ideally overnight. Use the parchment paper to lift the bars out of the pan and slice them into squares using a sharp, warm knife.