Using a large sharp knife, cut the top and bottom off of the watermelon flat and save the ends for later use. Turn the watermelon upright and carve away the rind in downward strips until all of the green and white pith is removed and you're left with a red, ripe center. Carve away small pieces from the sides and top until the center is roughly cake-shaped.
In a large mixing bowl, beat the heavy cream on high speed with an electric hand mixer until it begins to thicken. Add the sugar one tablespoon at a time, and then the vanilla bean seeds. Beat until the mixture is thick and spreadable, about 3 minutes.
Transfer the watermelon to a cake stand or serving plate and pat the outside with paper towels to remove excess water. Spread the whipped cream over the entire surface of the watermelon. Use a melon baller to cut round pieces of melon from the two reserved ends. Place them on the top center of the cake and garnish with mint sprigs, if using. Store the cake in the refrigerator.
Make it veganCoconut Whipped Cream2 (14 ounce) cans full-fat coconut milk, refrigerated overnight1/4 cup (50g) fine granulated sugar1 teaspoon vanilla extractScoop the coconut solids from the chilled cans and save the liquid for another use. Place the coconut solids in the bowl of an electric mixer fitted with the whip attachment. Beat on medium speed while adding the sugar gradually. Increase the speed to high and beat until fluffy. Beat in the vanilla extract. Use immediately to cover the watermelon.