First of all, prepare the cream (so that it has time to cool down well): put sifted flour, sugar and egg yolks in a saucepan and mix, then add the milk.
Start cooking over a low flame, stirring constantly, cooking until the cream has thickened (it will take about 10 minutes).

While the cream is still boiling, stir in the chopped chocolate, stirring until it melts completely, then transfer the cream to a bowl, cover with cling film and let it cool.

In the meantime, you can proceed to prepare the choux pastry: sift the flour, then put the water, butter and salt in a thick-bottomed pot and bring to a boil, then remove from the heat and add the flour all together, stirring quickly.
Put it back on the heat and stir until the dough comes off the walls.

Move the mixture into a bowl and let it cool well, then add the eggs, one at a time, adding the second only when the first is perfectly incorporated.

Transfer the choux pastry into a sac-à-poche with a star-shaped spout and create caps and stems of the mushrooms on a baking sheet lined with parchment paper, spacing them a little from each other (remember that the hats and stems must be in the same number): for the hats, make rounds of about 4 cm; for the stems, make sticks that are narrower on one side (because they will have to enter the hats), about 6 cm long.
Bake for about 25 minutes at 200 ° C, in a preheated convection oven, then turn off the oven, open the door a little and let it cool down.

When both cream and cream puffs are completely cold, you can assemble your mushrooms: put the cream in a pastry bag with a thin spout, pierce the bottom of the hats with the spout and fill them with cream, then insert the legs into the little hole left. from the sac-à-poche.
As they are ready, arrange the mushrooms on a clean surface and finally sprinkle them with powdered sugar and unsweetened cocoa.

The choux pastry mushrooms are ready, you just have to serve them.