Cut about 2 inches off both ends of the watermelon to create flat, stable surfaces. Stand the watermelon upright and carefully remove the rind by cutting downward in curved sections, following the watermelon's natural shape. You want a smooth, round 'cake' of pure watermelon flesh. Pat the surface completely dry with paper towels.

In a large chilled bowl, whip the heavy cream until soft peaks begin forming. Gradually add powdered sugar and vanilla extract, continuing to whip until stiff peaks form. The cream should hold its shape when you lift the beaters.

Place the prepared watermelon on your serving platter. Using an offset spatula or large spoon, spread the whipped cream evenly over the entire surface, creating smooth, cake-like frosting.

Arrange fresh berries in attractive patterns across the cream surface. Scatter mint leaves and sliced almonds for color contrast and added texture.