Mix flour and salt in a large mixing bowl. Add ice cubes to a measuring cup and fill with ½ cup cold water. Set aside.

Add fats: Grate frozen butter into the bowl, or cut it into very small pieces. Add chilled shortening and use a pastry blender or fork to cut the fats into the flour until well combined and resembles coarse crumbs.

Add ice water a spoonful at a time, mixing until the dough begins to come together into a ball. Be careful not to over mix, and you may not need all of the water. Gently mold the dough into a ball.

Divide dough into 2 pieces and press each to flatten into a disk.

Refrigerate: Cover dough discs with plastic wrap and refrigerate the dough for 2 hours (or freeze for 30 minutes). Refrigeration is mandatory as you need the fats in the pie dough to be cold when it's rolled out and placed in oven).

Roll out dough on a floured countertop. If you're new to pie making, it may be helpful to use this method: dampen a large cloth and lay it flat on your countertop, with a large piece of parchment paper over it (the damp towel will keep the parchment from moving). Lightly flour the parchment and dough and roll the dough into a large circle, about 1-inch larger then the diameter of your pie dish. Always start at the center of the crust, and roll outwards.

Place in pie pan: Gently turn the dough into your pie dish and remove the parchment paper. Settle it smoothly into the bottom and sides of the pan. Trim and crimp the edges of the crust.

Cook Chicken: Season chicken with salt and pepper. Add the chicken to a large saucepan and cover it with water. Bring it to a simmer and poach until it's just barely cooked through - then remove the chicken to a plate. Allow it to cool for a few minutes before chopping into bite-size pieces. Reserve about 1 3/4 cups of the water in a measuring cup, and discard the rest.

Chicken Pot Pie Filling: Add onions, celery and butter to the saucepan and cook for a few minutes, until soft and translucent. Stir in the flour, salt, pepper, garlic powder, bullion paste and celery seed. Slowly stir in the reserved water and milk. Simmer over medium-low heat until thick.

Add chopped chicken and frozen vegetables. Taste and season with more salt, pepper, bouillon or garlic powder if needed. Allow mixture to cool (so that's it's not hot when poured into the pie crust).

Preheat oven to 425 degrees F.

Roll out bottom pie crust: Remove one pie dough from the fridge and place on lightly floured counter. Use a rolling pin to roll dough into a large circle, about 12 inches in diameter. Place in 9 inch pie pan.

Assemble: Add cooled chicken pot pie filling. Roll out second pie crust and place over the top of the filling. Seal edges of the two crusts together and crimp them, if desired. Use a sharp knife to make a small slit in the top pie crust to allow steam to escape.

Egg wash: Whisk 1 egg with 1 tablespoon milk and brush a very thin layer over the top pie crust before baking.

Bake for 40-50 minutes, or until the top crust is golden brown and filling is bubbly. If you find the pie crust is browning too quickly, cover it with a piece of aluminum foil.

Rest and serve: Allow chicken pot pie to cool for at least 15-20 minutes before serving, to allow it to set up.