Remove the meat from the fridge and let sit for 20 minutes.
Prepare your peppercorns: Set your peppercorns on a cuttingboard and, little by little, using the flat side of a large chef's knife, press a (manageable!) cluster of peppercorns until they’re crushed. Transfer to a pie dish or similar vessel.
Lightly season both sides of the meat with about 3/4 teaspoon salt total. (Be conservative, as you’ll be preparing a pan sauce from the meat drippings later, and you can always add salt then.) Dredge the meat in the peppercorns, coating both sides.
Heat the oven to 425°, then heat the oil in a large cast iron skillet over medium-high. Once the oil is rippling hot — it should sputter if you flick a peppercorn into it — place the meat in the pan and brown on both flat sides, about 4 minutes per side. (If you have a sizable fat cap — a thick layer of white, chewy fat — on either of the smaller sides, sear them off until browned, 30 seconds to 1 minute per side.) If possible, take the temperature of your meat before adding it to the oven, so you have a sense of how long it’ll need to cook.)
Transfer the pan to the oven and cook to desired doneness, about 3 minutes for medium-rare (the temperature should be about 132 degrees). Remove the pan from the oven and transfer the meat to a cutting board to rest.
Add the shallot to the skillet and cook over medium-high until softened, stirring frequently, about 1 minute. Turn off the burner, then pull the hot skillet off the heat. Add the Cognac. (If the reaction between the heat and alcohol produce a flame, don’t panic! It will subside in a minute.)
Return to the stovetop, stir to deglaze and cook over medium high 1 to 2 minutes, until the liquid evaporates. Add the stock and cook until thickened and saucy, 6 to 8 minutes.
When the sauce is almost done reducing, slice the steak crosswise into 1/2-inch slices.
Once the sauce has reduced until thick enough to coat a spoon, pull it off the heat. Whisk in the butter, then the heavy cream (if using) and season with salt and pepper. Drizzle the sauce on a serving plate and transfer the steak on top. Serve immediately.