Spread out the puff pastry with parchment paper on a cookie sheet. Coat with the egg yolk, then use a fork and poke several holes to the top layer, sprinkling evenly with powdered sugar. Bake for 10 minutes in a preheated regular oven at 200°C.
Remove the puff pastry from the oven and let cool completely.
Let the gelatine leaf soak in bowl of cold water for 10 minutes.
Beat the coffee yogurt with 100 ml cream in a bowl until a compact and creamy texture emerges.
Squeeze out the gelatine, then place in a bowl with two tablespoons of cream. Heat the mixture on a low stove setting. Beat the leftover cream, and combine with the gelatine and then the coffee yogurt mixture.
Chill the mousse in the meantime.
Cut 10x5 cm large rectangles out of the baked puff pastry.
Fill a piping bag with the mousse and spray lots of little flakes on each rectangle, then sprinkle with some bitter cocoa before serving.