AT LEAST 30 minutes (and up to 1 hour) before you plan to cook, remove the beef filets mignon steaks from the refrigerator. Sprinkle all sides LIBERALLY with Kosher salt. Allow the beef to rest undisturbed on the counter.
Set a non-stick saucepan on the stove top over very low heat and warm the heavy whipping cream slowly. It needn't be hot, simply warmed.
Use a spice grinder, mortar and pestle, or plastic baggie and a meat mallet to coarsely crush the peppercorns.
When you are ready to begin cooking, pour the crushed peppercorns in a pie plate. Take each steak and press it into the peppercorns being sure to coat all sides.
Place a large cast iron skillet atop the stove and add the unsalted butter followed by the olive oil. Heat until they're melted, golden brown, and almost at the smoking point. Add the filets and allow the steaks to sear undisturbed for 4 minutes per side for medium rare. Up the time by 1 minute per side for medium.
Remove the steaks to a plate and wrap with foil to keep them hot and allow the juices to redistribute, 5 to 10 minutes.
Remove the cast iron pan from the heat. Blot the fat from the pan using a paper towel but leave the browned bits and the abandoned peppercorns. Hold the AWAY FROM YOUR OVERHEAD CABINETS. Add the Cognac all at once. Tilt the pan slightly. Light a long match (or click lighter) and wave it just above the surface of the Cognac. The vapors in the alcohol will ignite. Hold the pan as you swirl and swish the liquid. The flames will extinguish once the alcohol has burned off. NOTE: SEE MY VIDEO DEMONSTRATION IN THIS POST FOR HOW TO INGNITE THE COGNAC
Replace the cast iron to the heat and turn the heat to high. Pour off any of the juices from the steaks into the Cognac. Add the warmed heavy cream and whisk as the mixture comes to a boil. Reduce heat slightly and thicken the mixture, about 5 or 6 minutes.
Add an additional tablespoon of Cognac to the now thickened sauce, whisk to combine, and off the heat. Add the steaks back to the pan and spoon the sauce over top. Serve immediately.