Step 1: Prepare the Chocolate Ganache - In a double boiler or microwave-safe bowl, melt the 2 cups of semi-sweet chocolate chips with 2/3 cup of heavy whipping cream. Stir until smooth and silky. Allow it to cool slightly.
Step 2: Whip the Cream - In a large mixing bowl, combine 3 cups of heavy whipping cream, 1 cup of powdered sugar, and 1 teaspoon of vanilla extract. Using an electric mixer, whip the mixture until stiff peaks form, about 3-5 minutes.
Step 3: Combine Ganache and Whipped Cream - Once the ganache has cooled slightly, gently fold it into the whipped cream mixture, being careful not to deflate the whipped cream.
Step 4: Assemble the Cake - In your 9x13 inch baking dish, start by placing a layer of chocolate graham crackers at the bottom. Spread a layer of the chocolate filling over the crackers, then repeat until you reach the top, finishing with a layer of filling.
Step 5: Chill - Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight.
Step 6: Serve - Once chilled, slice the cake into squares and serve. Garnish with whipped cream or chocolate shavings if desired.