Preheat the oven to 200°C. Butter a baking dish with softened butter, then add the chicken and season with salt and pepper. Roast the chicken for 20 minutes, then allow to rest for 15 minutes. Pick the meat off the bone and chop it roughly.
Meanwhile, heat the stock. Slice the mushrooms and fry in 50g of butter in a large saucepan. Sprinkle the flour over the mushrooms and stir to coat evenly. Gradually pour in the hot stock and cream, stirring to incorporate. Simmer gently until thickened. If too thick, stir in a little extra stock. Fold the chicken meat and herbs through the mushroom mixture and season with salt and pepper. Refrigerate until chilled.
To make the pastry, dice the butter, then pulse with the flour in a food processor until the mixture resembles fine breadcrumbs. Add the sour cream and continue to pulse until the dough starts to incorporate into a ball. Wrap the dough in plastic film and refrigerate for 20 minutes. Roll the chilled pastry out until 5mm thick and cut rounds to line 6 individual pie dishes, taking the pastry over the edge of each to form a lip. With the remaining pastry, cut out 6 smaller rounds – these are the pie lids. Fill the pastry cases with the chicken mixture. Whisk the egg and milk together and brush this over the pastry ‘lip’. Put the lids into position and press around the edges with a fork to seal. Using a sharp paring knife, trim the pastry and, working from the centre to the edge, decoratively score the top of each lid.
Refrigerate the pies while preheating the oven to 220°C. Brush the tops of the pies with the egg wash again and bake for 20 minutes. Reduce the heat to 180°C and bake for a further 25 minutes. Allow the pies to cool slightly before turning out. Serve hot, cold or chilled with onion confit or Green Tomato Chutney.