At least 1 hour, or up to the day before filling is prepared, roll puff pastry out on a lightly floured surface to a square, 3/16-inch thick. Place on a baking sheet, wrap tightly with plastic, and chill for 1 hour.
Remove chilled dough from refrigerator and place on a cutting board. Invert a bowl or place a lid that is 2 1/2 to 3 inches larger in diameter than a 3 quart round casserole or souffle dish directly onto the dough. With a sharp knife cut around the bowl or lid so that you have a perfect circle. Return the circle of dough to the baking sheet, wrap tightly with plastic and chill for at least 2 hours.
Place a rack in lower 2/3 portion of oven; heat oven, preferably convection, to 425 degrees F.
Melt 5 tablespoons butter in a large heavy-bottomed pot over medium-high heat. Add onion, potatoes, and carrots. Cook, stirring occasionally, until the potatoes and carrots are fork tender, 10 to 12 minutes. Add mushrooms, cooking until mushrooms are heated through. Add cognac, and cook for 30 seconds. Add flour, and cook, stirring for 1 minute. Pour in chicken stock and milk. Bring to a simmer, cook until thick and bubbly, stirring constantly, 2 to 3 minutes. Stir in chicken, peas, parsley, and thyme. Season with salt and pepper. Transfer to 3-quart casserole or souffle dish. Transfer to a rack to cool slightly, about 5 minutes.
In a small bowl whisk together the egg, yolk, and 1 teaspoon water. Remove dough circle from refrigerator and brush surface evenly with egg wash. Invert dough and place directly over the casserole, pressing lightly to seal overhanging crust to side of dish. Brush top surface of dough with egg wash. With a1/2-inch large round pastry tip, cut a small circle in the center of the pastry, removing the pastry circle and insert the pastry tip, narrow side down, (Ateco #7) into cut out hole. This allows the steam to vent.
Transfer casserole to a parchment lined baking sheet. Bake for 10 minutes; cover loosely with aluminum foil to prevent crust from burning, continue baking for 25 minutes
1 pound all-purpose flour, accurately weighed
1 pound (4 sticks) cold unsalted butter, cut into small pieces
1 teaspoon salt
1 cup heavy cream (or 1/2 cup heavy cream mixed with 1/2 cup ice water)
In the bowl of a food processor or using the flat paddle of an electric mixer, mix 1/2 cup flour with the butter until very smooth. Shape the mixture into a flat square 1-inch-thick, wrap well in plastic, and chill for at least 30 minutes.
Combine the salt with the remaining flour in a large mixing bowl and add the cream (or cream and water). Mix the dough well by hand or with an electric mixer; the dough will not be completely smooth but it should not be sticky. Shape it into a flat square 1 1/2 inches-thick, wrap in plastic, and chill, at least 30 minutes.
Remove the flour dough from the refrigerator. On a lightly floured board, roll the dough into a rectangle twice as long as the butter dough. Place the butter dough in the center, fold up the ends to completely encase the butter dough, and seal the edges by pinching them together. Wrap well in plastic and chill for at least 30 minutes, so that the dough achieves the same temperature throughout.
Remove the dough from the refrigerator and, on a lightly floured board; roll it out into a large rectangle approximately 1/2-inch-thick. Fold the dough into 1/3's, aligning the edges carefully and brushing off any excess flour. The object is to ensure that the butter is distributed evenly throughout so that the pastry will puff evenly when baked. Wrap the dough and chill it for at least 30 minutes. This completes 1 turn.
Repeat this process 5 more times; classic puff pastry gets 6 turns, creating hundreds of layers of butter between layers of the flour dough (729 to be exact). Use as little flour as possible when rolling out the dough, and always brush off any excess. Remember to let the dough rest for at least 30 minutes in the refrigerator between turns, or 15 minutes in the freezer. This chilling makes the rolling out much easier and it keeps the layers of butter of equal thickness.
By the sixth and final turn, the dough should be very smooth, with no lumps of butter visible. Wrap the pastry in plastic wrap and refrigerate until ready to use (up to 2 days), or freeze for future use.
Yield: about 2 pounds
Poached Whole Chicken:
1 whole chicken weighing 4 1/2 to 5 pounds
9 to 10 cups chicken stock
Place chicken and stock to cover in a medium saucepot. Bring to a boil, reduce heat, and simmer gently until chicken is cooked through, about 45 minutes.
Using tongs, transfer chicken to a plate, set aside until cool enough to handle. Pull the meat from the bones in large pieces, discarding skin and bones. Reduce liquid to 2 cups for Chicken Pot Pie.
Yield: about 5 cups cooked chicken