Cut the watermelon into thick slices, about 1-2 inches thick, and trim the rind to create a cake-like shape.

In a mixing bowl, whip the heavy cream until soft peaks form. Gradually add powdered sugar and vanilla extract, and continue to whip until stiff peaks form.

Place one watermelon slice on a serving plate. Spread a layer of whipped cream on top, then add another watermelon slice. Repeat the layers until all slices are stacked.

Spread the remaining whipped cream over the top and sides of the cake. Decorate with mixed berries, banana slices, and mint leaves.

Slice and serve immediately, or refrigerate for up to 2 hours before serving.