First, prepare all the ingredients in the amounts listed. Take the mascarpone out of the refrigerator 30 minutes before using so it comes to room temperature.

Cut the watermelon and remove the seeds. Dice it into cubes; set some aside for the semifreddo and some for decorating at the end.

Add the mascarpone, powdered sugar, Cointreau, and watermelon to a blender. Blend until you get a light, creamy mixture. Once the mixture is smooth and fluffy, pour the semifreddo into serving cups. Let it chill for a few hours. When ready to serve, top with sweetened whipped cream.

Finally, add the reserved diced watermelon on top.