Pour the cold heavy whipping cream into the chilled bowl. Add powdered sugar and vanilla if using. Beat with a hand mixer or stand mixer on medium-high speed until soft-to-medium peaks form and the cream is thick and spreadable. Do not overbeat or it will turn grainy.
Rinse, hull, and thinly slice the fresh strawberries. Pat them dry with a clean kitchen towel or paper towels so they don’t add too much extra moisture to the cake.
In a glass rectangular baking dish (about 9×13 inches), spread a thin layer of whipped cream over the bottom. This helps anchor the first layer of graham crackers and keeps them from sliding around.
Arrange a single layer of graham crackers over the whipped cream, breaking pieces as needed to fit the corners and edges of the dish in a mostly even layer.
Spread a generous layer of whipped cream over the graham crackers, using about 1/3 of the cream. Smooth it out with a spatula so all the crackers are covered.
Scatter an even layer of sliced strawberries over the whipped cream, using about 1/3 to 1/2 of the berries for this layer, depending on how berry-heavy you like it.
Repeat the layers: graham crackers, whipped cream, and strawberries, until you have 3 layers of graham crackers and 3 layers of whipped cream. Reserve a good handful of the prettiest strawberry slices for the very top.
Finish with a final layer of whipped cream spread all the way to the edges so no crackers peek through. Arrange the reserved strawberry slices on top in a simple pattern or just scatter them evenly for a rustic look.
Cover the dish tightly with plastic wrap or a lid and refrigerate for at least 4 hours, but preferably overnight. The longer it chills, the more the graham crackers soften into a cake-like texture and the flavors meld together.
