Trim the filets if necessary, removing all but the central true filet part, as well as the tough "silverskin." Salt and pepper lightly.
Heat a heavy skillet over medium heat. Add the butter and turn the pan to distribute and melt it quickly. Put the steaks into the skillet. Scatter the chopped shallots around the steaks.
Cook the steaks for three to five minutes per side. The steaks are ready to be turned when they are mostly unstuck from the pan. That doesn't always happen, but it's a good guide. If the steak is really stuck down there it's probably not ready to turn yet. If the steaks are thick, cook them on their sides as well as on their faces.
Add the brandy to the pan and bring to a boil. (Be careful: it may catch fire, which is all right for flavor but potentially dangerous.) With a whisk, dissolve the browned bits of meat in the brandy as it boils.
When the brandy is almost boiled away, add the cream and cracked peppercorns (and the demi-glace, if you have it). Bring to a light boil and cook, agitating the pan to mix the ingredients until reduced by about a third.
Place the steaks on warm plates and spoon on the sauce.