Put oven rack in middle position and preheat oven to 425°F. Pat venison dry and sprinkle with black pepper and 1/4 teaspoon salt. Heat oil in an ovenproof 12-inch heavy skillet over moderately high heat until just smoking, then brown venison on all sides (except ends), about 3 minutes total. Transfer skillet with venison to oven and roast until thermometer inserted diagonally 2 inches into center registers 115°F, 4 to 5 minutes. Transfer venison with tongs to a plate and let stand, loosely covered with foil, 5 minutes (temperature will rise to 125°F for medium-rare). While meat stands, add shallot to skillet (handle will be hot) and cook over moderate heat, stirring, until golden, about 2 minutes. Add Madeira and deglaze skillet by boiling, stirring and scraping up brown bits, until liquid is reduced by half, about 1 minute. Add demi-glace and boil, stirring, until sauce is slightly thickened, about 1 minute. Stir in peppercorns, butter, remaining 1/4 teaspoon salt, and any meat juices that have accumulated on plate, then remove from heat. Cut venison into 1/2-inch-thick slices and serve with sauce.