Steak Diane really needs a tender cut, and with venison that means tenderloin or backstrap. The best way to cook this is with a large piece of backstrap that you then cut into medallions right before you serve. If you have regular medallions, it will still work. While it is important to use heavy cream for this recipe (lighter creams will separate), it is not that important to have fancy brandy for this recipe -- just use something you would drink, OK?
Heat the butter in a large saute pan over medium-high heat for about 90 seconds. Pat the venison dry with a paper towel and cook it on all sides. Turn the heat to medium so the butter doesn't scorch, and take your time. It should take about 8 to 10 minutes or so to get a nice brown crust on the venison without overcooking the center. Remove the venison, tent loosely with foil and set aside.
Turn off the heat and let the boiling subside. Stir in cream until the sauce is as light as you like. Don't let the sauce boil again or it could break.
Slice the venison into thick medallions. If you find you have not cooked it enough, let the meat swim in the sauce for a few moments to heat through. If the venison is to your liking, pour some sauce on a plate and top with the meat. Garnish with some chopped herbs. Chives are traditional, but basil and parsley are also nice.
Looking for another classic to make with venison tenderloin? Try Venison with Cumberland Sauce.