In a large mixing bowl, combine the flour, salt, and baking powder. Add in the butter and milk and mix until just combined. The dough will be fairly wet, but should still hold together. Turn the dough out onto a floured surface, sprinkle the dough with flour, and pat it into a disk 1-inch thick. Wrap in plastic wrap and chill in the refrigerator while making the filling for the pot pie.
Prepare Chicken Pot Pie Filling
If not using leftover cooked chicken, parboil the chicken by placing the raw chicken breasts or thighs into a large stock pan. Cover with cold, tap water and add ½ teaspoon kosher salt to the water. Over high heat, bring the water to a boil and then reduce the heat to medium-low and simmer for 15 minutes, or until chicken is cooked through. Allow the chicken to cool slightly and then cube or shred into bite-sized pieces.
While the chicken is simmering, place cubed potatoes and sliced raw carrots, along with 1 teaspoon kosher salt into a medium stock pan and cover with cold tap water. Bring the water to a boil and then reduce the heat to medium-low and simmer the potatoes and carrots until fork-tender, about 15 minutes. Once cooked, drain the vegetables and set them aside.
Once the potatoes, carrots, and chicken are cooked, prepare the sauce, by melting ¼ cup of butter over medium heat in a large stock pan. Once the butter is melted, whisk in the flour and cook for 1 minute, to allow the flour to toast.
Slowly add in ½ cup milk and ½ cup chicken stock, whisking well to incorporate. Bring the mixture to a boil, while whisking constantly.
Once boiling, whisk in the remaining 1 cup of chicken stock and ½ cup of milk. Season with 1 teaspoon each of seasoned salt and pepper. Bring the mixture back up to a boil, whisking often. Once the mixture comes to a boil again, turn the heat to low and simmer for 5 minutes. Turn off the heat.
Stir in the cooked chicken, cooked potatoes, carrots, and frozen peas, and gently stir to combine.
Assemble and Bake
Preheat the oven to 350 degrees F.
Pour the chicken and vegetable mixture into a buttered 2-quart or 9x13 casserole dish.
Remove the biscuit dough from the refrigerator and place it on a floured surface. Roll out the biscuit dough to ¼-inch thick and cut into long strips. Place strips lengthwise across your dish and then width wise (a lattice pattern.)
Whisk the egg together with 1 tablespoon of water and brush over dough strips. Sprinkle with sesame seeds and poppy seeds, if using.
Bake at 350 degrees F for 30-40 minutes until dough is golden brown and filling is bubbling hot.
Storing: This chicken pot pie will last for 3-4 days in the refrigerator in an airtight container. It does not freeze well, due to the potatoes and homemade sauce. Pot Pie Crust: Feel free to use refrigerated pie crust or puff pastry in place of homemade biscuit topping. You can also use refrigerated biscuit dough, but it is a bit harder to roll out thinly and slice for the lattice topping. Egg-Free: Instead of brushing the pastry with an egg wash, use milk or melted butter. Using Frozen Carrots: Instead of parboiling raw carrots, use frozen sliced carrots and add to the filling when adding the frozen peas. Prepping in Advance: You can prepare the components of the chicken pot pie up to 3 days prior to baking. Mix the cooked carrots, potatoes, peas, and chicken into the sauce and store them in an airtight container. Prepare the biscuit dough as directed and wrap well in plastic wrap and refrigerate for up to 3 days. While the oven is preheating, assemble the pot pie by pouring the chicken mixture into a casserole dish, and then roll out the biscuit topping and add to the pot pie mixture as directed. Brush with an egg mixture, sprinkle with sesame seeds and bake. Keep in mind you may need to increase the baking time by 10 minutes, as the filling will be cooler after refrigeration.