Peel the mango, cut half of it into small cubes ( size the mango so that the cubes can go out the nozzle of the piping bag you will use) and the other half in thin slices.
Cut 6 of the prawn tails in cubes of the same size.
Take the cream off the fridge and make a steady 'chantilly' (whipped cream - no sugar added of course !) with it and season it with salt and pepper. With a wooden spatula, carefully incorporate the mango cubes, the prawn cubes and the chantilly.
Fill, using a piping bag, 6 nice glasses or sundae dishes with the cream (and mango and prawn cubes) and on two sides of each glass, put 2 prawns and on top at the middle, 1 slice of mango topped with 2 tarragon leaves.