Pre-heat the oven to 200°C/180°C fan and take the Yorkshire Puddings out of the freezer to allow them to defrost.
Sauté the onion, carrot, parsnip and leeks with the butter in a pan over a medium heat until the veg begins to soften. Add in the chicken and mushrooms and continue to fry until the chicken is cooked through.
Whilst the vegetables and chicken are cooking, prepare the creamy sauce. Add the butter beans (including the water from the can) and the vegetable stock pot to a food processor and blitz until smooth.
Stir the pureed butter beans into the pan, season to taste if needed, bring the mix to the boil and then simmer for 5-10 minutes.
Whilst the mixture is simmering, roll out 6 of the Yorkshire Puddings with a rolling pin so that they are flat and set aside. These will form the lids of the pies. Once the mixture has simmered, begin to fill the 6 Yorkshire Puddings. Using a spoon, fill each Yorkshire Pudding to the top, pushing the mixture down and to the sides to fill them with as much filling as possible.
Place the filled Yorkshire Puddings in a greased oven-proof dish, place the flattened Yorkshire lids on each pie then brush all over with egg wash before placing in the oven for 20 minutes.
After 20 minutes, egg wash the pies again and bake for a further 10-15 minutes before serving.