Preheat an oven to 425ºF. Remove 8 frozen Vol-au-vent cases from the package and set aside.
In a medium saucepan melt the butter. Slice the button mushrooms and saute briefly in the butter.
Add the can of soup plus a half can of milk, stirring well to mix with the mushrooms as the mixture heats through.
When the oven is up to temperature, place the frozen vol-au-vent cases on a baking sheet and bake for the time on the packet.
When the soup and mushroom reach a simmer, add the cooked chicken and give it all a good stir. When the mixture is thoroughly heated, remove from heat and set aside.
Remove the vol-au-vent cases from the oven. Plate up by first flipping the tops off the pastry and filling the cases with a good ladle of the chicken and mushroom mixture then pop the top back on. Serve with a side vegetable of your choice.