In a small mixing bowl, whisk together the egg yolk, whole milk, and chives. Set aside.

Measure and add the dry ingredients and diced butter to the bowl of a stand mixer fitted with a paddle attachment.

Mix on low speed for 3 minutes until it reaches a sand like texture. It is very important that it remains a coarse texture at this point.

With the mixer on low speed, add the wet ingredients into the dry ingredients and mix for 1 minute or until just combined.

Remove the pie dough to a lightly floured surface and form it into a disk. Wrap the dough disk in plastic wrap and refrigerate until the filling is cooked. Preheat the oven to 350°F/175°C.

Cut the chicken into 1/2 inch cubes and set aside.

Preheat the pot to 325°F/160°C (medium) and melt the butter completely.

Cook the vegetables for 6 minutes, stirring frequently until softened.

Add the mushrooms, garlic and herbs and continue cooking for 2 minutes, stirring frequently, until the mushrooms begin to brown.

Reduce the temperature to 300°F/150°C (medium-low) and cook the flour for 1 minute and 30 seconds, stirring frequently.

Allow the brandy to reduce for 10 seconds while stirring continuously to scrape up any browned bits on the bottom of the pot.

Reduce the temperature to 220°F/105°C and simmer the chicken stock for 6 minutes, stirring frequently.

Continue simmering for 8 minutes or until the chicken is cooked, stir occasionally.

Fold the remaining ingredients into the filling.

Remove and discard the thyme and bay leaf. Transfer the filling to the casserole dish.

Remove the dough from the refrigerator. Using a rolling pin, roll out the pie dough on a lightly floured surface until it is about 1/4 inch thick and large enough to cover your casserole dish.

Lay the pie dough over your casserole dish and gently press the dough down against the filling and the edges of the dish. Using the rolling pin, roll over the top of the casserole, pressing firmly against the edges to trim away the excess dough. Decorate the pie to your liking. Using a fork or knife, prick the top to let air vent while baking.

In a small bowl, whisk the milk and egg yolk until combined. Brush a thin layer of the egg wash on the crust.

Place the pot pie in the preheated oven and bake for 30 minutes or until until the pie is a deep golden brown and the filling is bubbling. Depending on your oven, you may need to add 5 minutes.

Remove from the oven and serve immediatly.