Put the chicken stock and the bouquet garni in a pot, bring to a boil, and let infuse for about 10 minutes.
Cut the chicken breast into small dice.
Quickly rinse the mushrooms and halve or quarter them depending on size.
In a frying pan, melt the butter over medium-high heat, add the shallot, and cook for about 1 minute, stirring.
Add the flour and cook for about 1 minute, stirring with a whisk or wooden spoon, without browning it.
Pour in the chicken stock gradually and cook for 2 minutes, stirring.
Add the diced chicken and the mushrooms, and cook for 5–6 minutes, stirring occasionally.
Preheat the oven to 180°C and bake the vol-au-vent shells for 5–10 minutes (or as directed on the package), without letting them burn.
In a bowl, mix the cream with the egg yolk. Raise the heat under the sauce, add the egg-cream mixture, beating to combine, and lower the heat as soon as it reaches a boil; the sauce should be velvety and coat the back of a spoon. Season only with pepper, no salt.
Take the vol-au-vents out of the oven, fill them quickly, and serve without delay, as they cool fast.