1) A day before you plan to cook the pheasants, carefully loosen the skin; do this gently, as pheasant skin is extremely delicate and tears easily. Divide the garlic and herb salt evenly between two bowls; add the baking powder to one bowl and set that aside. Spread the plain garlic-herb salt underneath the skin, rubbing it over the meat. Keep in mind that you won’t be rinsing the pheasants to remove excess salt, so use only as much as you would typically use for seasoning (you won’t need all of it). Apply the salt–baking powder mixture on the skin over the outside of the birds (again, you won’t use it all).
2) Set the birds on a baking sheet lined with a wire rack and place it in the refrigerator, rearranging things so they get plenty of airflow. Let stand in the fridge, uncovered, for 24 hours. Dry brining the pheasant this way not only makes the meat juicy, it also dries out the skin to help it brown better.
3) The next day, remove the pheasants from the refrigerator 45 minutes to 1 hour before you plan to cook them. Preheat the oven on the convection setting to 225°F (or 250°F if your oven doesn’t use convection). Lightly grease a roasting pan with oil.
4) Using a paper towel, pat the pheasants dry and dust off excess salt from the top. Rub the softened butter under the skin over the meat and then over the skin on the outside of the birds. Use kitchen twine to truss the legs and place a few sprigs of herbs inside the cavity of each bird.
5) Lightly coat the shallots and grapes with oil, then season them with salt and pepper. Place the shallots and grapes in the greased roasting pan and nestle the pheasants on top, using the grapes to support the birds so they sit breast-side up without rolling over. Place an oven-safe meat thermometer in the tenderloin (the breast meat closer to the bone). Insert the thermometer vertically from the top of the bird downward. Transfer to the oven. Roast until the meat reaches 125°F.
6) Remove the roasting pan from the oven — but don’t take the thermometer out of the pheasant — and raise the oven temperature to 500°F convection (or as high as your oven will go if it doesn’t have a convection setting). This may take 20 to 30 minutes, and you may notice the internal temperature of meat going down, but don’t worry — it’ll go back up in a hurry once it’s back in the oven.
7) Baste the pheasants with more butter. When the oven reaches 500°F, return the pheasants to the oven and roast until the breast reaches 155°F. Remove from the oven and transfer the pheasants to a cutting board to rest.
8) Meanwhile, place the roasting pan on the stovetop over high heat. Add the vinegar, stock and cream, and bring to a boil, stirring and scraping up any browned bits from the bottom of the pan, then reduce the heat to maintain a simmer. Cook until the mixture has reduced by half, about 5 minutes, then remove from the heat. Taste the pan sauce and season with salt and pepper as needed.
9) Carve each pheasant into serving pieces. Serve with the pan sauce drizzled over the top and the roasted grapes and shallots alongside.