Unroll the puff pastry, halve it lengthwise, and lay it with the paper in two rectangular baking tins (such as cake tins approx. 10 x 26 cm in size) so that the pastry has an edge approx. 2 cm in height. Press the dough down and prick all over with a fork.
Blind bake the pastry for approx. 20 mins. in the centre of an oven preheated to 220°C. Remove from the tin and place on a rack to cool.
Using the whisk on a hand mixer, mix the crème powder and 500 ml milk for approx. 1 min. at the lowest speed, then switch to the highest speed and mix for about another 3 mins. Whip 200 ml of cream to stiff peaks and fold it into the crème. Spread the mixture on the baked puff pastry.
Arrange the mango slices in a wave pattern on the crème and decorate with meringues and chocolate shavings.