Combine the raspberries and granulated sugar in small pot over low heat. Crush the berries with a potato masher or large spoon. Stir the mixture and continue to break down the fruit while bringing the mixture to a gentle simmer; about 4-5 minutes. Remove from heat and refrigerate until well-cooled.

(OPTIONAL - feel free to omit this step if you like a lot of raspberry seeds). Pass the raspberry puree through a food mills or coarse strainer to remove some or all of the seeds. Try to reserve as much juice and pulp as possible, then set aside.

Beat the whipping cream and cream of tartar in a large, non-reactive (and preferably chilled) bowl until set, but before it starts to form stiff peaks. Separate out 1/2 cup of the whipped cream and set it aside to mix with the raspberries. Add the vanilla, cocoa powder, and powdered sugar to the remaining cream and continue to mix/whip until stiff peaks form.

Combine the reserved 1/2 cup of whipped cream with the raspberry puree and set aside in the fridge. This raspberry cream mixture should be thinner and runnier than the chocolate cream.

Put a dollop of chocolate cream down on a medium size baking tray or cutting board (something you can fit in your fridge). Lay 4 whole (or 8 half) graham crackers edge-to-edge out on the cream, using it like mortar to keep them in place. You can lay the crackers down in any shape you like, but I like to work with a loaf-shaped rectangle. Spread about 1/3 of the chocolate cream evenly over the crackers. Add another layer of crackers, then a little under half of the raspberry cream. Repeat the process (crackers, chocolate, crackers, raspberry), then top off with the final layer of crackers. Break the two remaining graham wafers into quarters and place them along the outside edges of the cake to help hold in the filling, then use the remaining chocolate cream to cover the top of the cake and to thinly cover the sides (if your raspberry cream is particularly runny, you might want to add the side pieces after the first two layers in order to help keep the filling from spilling out on you). Drizzle the last little bits of raspberry cream over the top for a pop of colour.

If you like, you can use a fork or decorating tool to draw designs in the chocolate cream. Carefully cover the cake with plastic wrap (use a few toothpicks to hold it up off of the surface) and refrigerate for 12 hours or more. Serve cold and garnished with fresh raspberries.