Add the heavy cream, sugar and vanilla to a large bowl and whip for 2-3 minutes on high speed or until stiff peaks form. Then set aside for later.
Cut the ends of the watermelon off then set the watermelon up on its end.
Carefully slice the rind from off of the outside of the watermelon and form it into a cylindrical shape as best you can without cutting too much of the fruit off.
Lay the watermelon back down on its side then slice it in half.
Place one half of the watermelon on a cake stand then add a layer of the whip cream on top then place the other watermelon half on top.
Blot the outside of the watermelon with a paper towel to get rid of any excess moisture then cover the outside with the remaining heavy cream.
Add the strawberries, blueberries and raspberries to the top of the cake along with a few mint leaves then slice and serve.
Notes
This recipe makes 8 large slices. You can divide each slice in half for 16 servings.Storing: This cake should be served immediately for best results. The whipped cream frosting can be stored in the fridge for up to 3 days in an airtight container. Freezing: Due to the high water content in watermelon, freezing is not recommended as thawing will change the structure of the watermelon.