Cut the ends off of your watermelon with a very sharp knife. Set aside.
Set the cake upright. Cut off all of the green rind and white pith. Gradually cut off the longer sides until it resembles a circle.
Blot watermelon with paper towels to remove any excess moisture. Air dry in the fridge for about 1 hour.
Use a chilled bowl and beats to whip your cream until it begins to thicken. Add vanilla. Gradually add sugar a little at a time. Continue mixing until it forms soft peaks. Spread whipped cream onto the melon cake using an offset spatula. Top with berries if desired.