Using a large sharp knife, cut a small slice off the top and bottom of the watermelon so it sits flat. Carefully cut away all the rind and white pith, following the curve of the fruit. You want a clean, round cylinder of red flesh.

Pat the watermelon completely dry with paper towels. This is crucial—any moisture will make the frosting slide right off. Place the watermelon on a serving platter.

In a large bowl, beat the softened cream cheese with an electric mixer until smooth. Add the powdered sugar and vanilla, and beat until combined.

In a separate bowl, whip the heavy cream to stiff peaks. Gently fold the whipped cream into the cream cheese mixture until smooth and fluffy. This is your frosting.

Using an offset spatula, spread a thin layer of frosting over the top and sides of the watermelon. This is the crumb coat—it catches any excess moisture. Refrigerate for 15 minutes to set.

Apply the remaining frosting in a thick, even layer. Use the spatula to create swirls or a smooth finish.

Immediately decorate the top with fresh berries, mint, coconut, or edible flowers. Press some berries gently into the sides if desired.

Refrigerate for at least 30 minutes before slicing. The cake should be served cold. To slice, use a sharp knife and wipe it clean between cuts.