Step 1: In a large, chilled mixing bowl, combine the cold heavy cream, cold mascarpone cheese (if using), powdered sugar, and vanilla extract. Beat with an electric mixer on medium-high speed until stiff peaks form. Be careful not to overbeat, which can cause the mixture to curdle. The cream should be thick and hold its shape.
Step 2: Arrange a single layer of graham crackers in the bottom of a 9x13 inch baking dish or a 9-inch springform pan, breaking them as needed to fit snugly. This will form the base layer of your cake.
Step 3: Spread about one-third of the whipped cream mixture evenly over the graham cracker layer. Use an offset spatula or the back of a spoon to smooth it out.
Step 4: Scatter about one-third of the mixed fresh berries evenly over the cream layer. Ensure good coverage for consistent berry flavor throughout the cake.
Step 5: Repeat the layering process: add another layer of graham crackers, followed by another third of the whipped cream, then another third of the berries. Continue until you have used all ingredients, making sure the top layer is cream.
Step 6: Cover the cake tightly with plastic wrap. Refrigerate for at least 6-8 hours, or preferably overnight (12-24 hours). This crucial chilling time allows the graham crackers to absorb moisture and transform into a cake-like texture.
Step 7: Before serving, remove the cake from the refrigerator. Garnish with the reserved fresh berries and, if desired, a light dusting of powdered sugar or a few fresh mint leaves. If using a springform pan, carefully remove the sides. Slice into servings and enjoy immediately.