In a medium bowl, whisk the yogurt to loosen it. In another medium bowl, beat the cream with 1 1/2 tablespoons of the sugar until it thickens slightly (it should not be thick enough to hold peaks). Fold the whipped cream into the yogurt.
In a blender or food processor, puree the mangoes, lime juice, and the remaining 1 1/2 tablespoons sugar. Add more sugar to taste, if desired. Fold the mango puree into the yogurt and cream mixture until they are partially combined.
Spoon the fool into 4 glasses and chill until ready to serve. Garnish with the dates and pistachios, if desired.
Recipe courtesy of Peter Berley, 2008