Mix half of the cream with the sugar and bring to the boil. Remove from the fire and stir in the gelatine that has swollen in the water. Pour into a bowl and stir in the buttermilk and the scraped vanilla pulp. Chill for twenty minutes, stirring occasionally.
Whip the rest of the cream and fold into the buttermilk cream. Pour the mixture into four oiled bowls, cover with cling film and refrigerate for at least two hours.
For the sauce, puree the mangoes and, if necessary, sweeten with powdered sugar.