Puree the raspberries using a blender or food processor.
Pour the heavy cream into a chilled bowl and, using an electric mixer, beat the cream until it begins to thicken. Stop the mixer, add the sugar, and continue to beat on high until firm peaks form when the mixer is lifted from the cream.
Gently fold 1/3 cup of the raspberry puree into the whipped cream. Do not mix too well, as you want swirls of the raspberry in the cream.
Pour a small amount of the remaining raspberry puree into the bottom of 6 martini glasses*. Mound about 3 large tablespoons of the fool onto the puree. (The recipe can be made to this point and chilled before dinner.) Top the fool with the fresh berries and mango and garnish with the mint sprig.
*Any stemmed glass with a wide mouth makes a dramatic presentation.