Notes: Rounds cut from the small end of the beef tenderloin are sold as filet mignon; substitute USDA prime or choice beef for the Kobe if desired. Start the potatoes first, then make the green peppercorn sauce; keep it warm over low heat while you cook the beef. Demiglace and stock bases are sold in some supermarkets, specialty food stores, and some cookware stores (such as Williams-Sonoma); reconstitute a stock base to the demiglace level to use in the sauce.
4 pieces Kobe-style beef tenderloin (cut from small end, each about 1 1/2 in. thick and 8 oz.; see notes), fat trimmed
Rinse beef and pat dry. Season all over with salt. Put cracked black pepper on a small, rimmed plate.
Heat olive oil and butter in a 10- to 12-inch frying pan over medium-high heat. When butter just begins to brown, press a flat side of each piece of beef into pepper to form an even crust (discard any leftover pepper); set beef, pepper side down, in pan and cook until browned on the bottom, 4 to 5 minutes. Turn pieces and cook until other sides are browned and a thermometer inserted into the center reaches 125° for rare, about 8 minutes longer, or 135° for medium-rare, 12 to 13 minutes.
Transfer beef to warm plates and let rest for 5 minutes. Mound rosemary roasted potato wedges alongside beef. Pass green peppercorn-brandy sauce to add to taste.