In a large, chilled mixing bowl, combine the cold heavy whipping cream, powdered sugar, and vanilla extract. Using an electric mixer fitted with the whisk attachment, beat on medium-high speed until stiff peaks form. The cream should hold its shape firmly when the whisk is lifted. Be careful not to overbeat — overwhipped cream will turn grainy and eventually become butter. Set aside.

Have your 9x13 inch baking dish ready. No need to grease or line it. Arrange a single, tight layer of graham crackers on the bottom of the dish, breaking crackers as needed to cover the entire surface edge to edge. This forms the foundation of your icebox cake.

Spread about one-third of the prepared whipped cream evenly over the graham cracker layer using an offset spatula or rubber spatula. Ensure a smooth, consistent layer from edge to edge.

Evenly distribute half of the sliced strawberries and half of the blueberries over the whipped cream layer. Reserve enough berries for the flag decoration on top.

Place a second layer of graham crackers over the fruit, pressing down gently. Spread another one-third of the whipped cream over the crackers, then add the remaining half of the strawberries and blueberries.

Add the third and final layer of graham crackers. Spread the remaining one-third of the whipped cream evenly over the top, creating a smooth, flat surface. This is your canvas for the flag design.

In the upper-left corner of the dish, arrange blueberries in a tight, neat grid pattern to represent the stars (the canton of the flag). Then, arrange sliced strawberries in horizontal rows across the rest of the cream surface to create the red stripes of the flag.

Cover the baking dish tightly with plastic wrap. Refrigerate for at least 4 hours, or preferably overnight. The chilling period is essential — it allows the graham crackers to absorb moisture from the whipped cream and soften into a tender, cake-like texture. Slice into portions and serve cold.