Preheat a cast-iron skillet to high. Coat the steaks with the oil and season with salt. Season the steaks on one side with the pepper. Add the steaks to the skillet pepper side down and cook, turning once, for 4 minutes. Transfer to a plate and tent with foil. Wipe out the pan.
Add the shallots and 1 teaspoon butter to the skillet and cook over medium heat until softened, 1 to 2 minutes. Remove from the heat and stir in the cognac. Simmer over medium heat until reduced by half.
Meanwhile, transfer any steak juices to a small bowl. Add the evaporated milk and whisk in the cornstarch. Whisk the mixture into the skillet and bring to a boil, whisking, until slightly thickened, about 2 minutes. Whisk in the remaining 1 teaspoon butter; season with salt. Cover to keep warm.
Thinly slice the steaks and toss in the sauce to coat. Arrange the steak on the toasts and top with any remaining sauce and the parsley.