1. Preheat the oven to 200°C (400°F).
2. Cut the chicken breasts (or leftover chicken) into very small pieces (about 1.5 cm). Cut the mushrooms into the same size. In a sauté pan, cook the chicken in 20 g of butter. Add the mushrooms, a little more butter, and cook everything over low heat. Season with salt and pepper.
Note: If using leftover chicken, add it at the same time as the mushrooms.
3. Bring water to a boil for the rice.
4. Start preparing the sauce:
In 1/2 liter of boiling water, dissolve the chicken bouillon cube. Once well dissolved, set aside.
5. In a saucepan, melt the 50 g of butter. Once it is hot, add the flour all at once. Stir gently and let it brown for a few moments without stopping stirring.
Still on the heat, gradually pour in the bouillon while stirring, and let it thicken. Add the lemon juice, salt, and pepper.
Add the chicken and mushrooms to the sauce.
6. Heat the vol-au-vent shells in the oven for about ten minutes (be careful not to burn them).
Fill them with the filling and serve with rice.