Process parsley, thyme, rosemary, garlic, 2 teaspoons salt, and 1 teaspoon pepper in food processor until paste forms. Reserve 1 teaspoon herb past for sauce. Combine 2 tablespoons herb paste with oil in small bowl. Set aside remaining herb paste for under skin.

Adjust oven rack to middle position and heat oven to 450 degrees. (But for my bigger chickens this was too hot; the outside would have burned before they were cooked all the way through. So I used 425 with the convection setting, which meant that it was really 400.) Pat chickens dry with paper towels. Rub remaining herb paste under skin of each chicken, making sure to coat breast, thigh, and leg meat. (Hah. Easier said than done. See picture, where you can tell that the herb paste is basically just over the breasts and thighs.) Rub herb-oil paste over outside of each chicken. Tuck wings behind back, and tie legs together with kitchen twine. (Which I didn't do. too fiddly!) Transfer chickens to platter. Cover and refrigerate 1 hour.

Arrange chickens 2 inches apart, breast side down, on V-rack set inside large roasting pan. Roast until thigh meat registers 135 to 140 degrees, about 45 minutes. Remove chickens from oven and, using wad of paper towel, flip breast side up (meat that was facing inside should now be facing outside). Pour 1 cup water into roasting pan. Return chickens to oven and roast until thigh meat register 170 to 175 degrees, at least 45 minutes more for this size chicken. Transfer chickens to carving board and let rest 20 minutes.

Whisk cornstarch with remaining water in small bowl until no lumps remain. Pour pan juices and any accumulated chicken juices into liquid measuring cup; skim fat. Transfer 1/2 cup defatted pan juices to medium saucepan. (Why on earth would you only use 1/2 cup of the lovely pan juices? Just use all of it. But do be aware that these juices are going to be quite salty. Also, while you want the nice brown bits, you don't want those ugly scummy protein blobs, so skim those out while you're at it.) Add broth and wine and bring to boil. Reduce heat to medium-low and simmer until sauce is slightly thickened and reduced to 1 1/4 cups, 8 to 10 minutes. Whisk in cornstarch mixture and simmer until thickened, 3 to 5 minutes. Off heat, whisk reserved herb paste (and the butter if you're using it). Taste to see if it needs salt or pepper; it almost certainly won't need salt. Carve chickens and serve, passing sauce at table.