Using tenderloin, a leaner cut of steak, allows you to indulge in a flavored butter topper. You'll only use half of the cognac butter, but make all of it--cutting the amount in half doesn't work as well. Use the leftover butter within the week, on pasta or over fish, or freeze up to one month; bring to room temperature before serving.
1. To prepare cognac butter, melt 1 1/2 teaspoons butter in a small nonstick skillet over medium-low heat. Add shallots; cook 2 minutes or until tender, stirring shallots occasionally. Carefully stir in cognac; cook 1 minute or until liquid is reduced by about one-third. Remove from heat; cool.
2. Place 6 1/2 tablespoons butter in a small bowl; stir in cooled cognac mixture, 1 tablespoon thyme leaves, and 1/8 teaspoon black pepper. Cover and chill 10 minutes. Divide butter mixture in half. Scrape each half of butter mixture onto a piece of plastic wrap; shape each portion into a 4-inch-long log. Wrap each butter log in plastic wrap; refrigerate or freeze 1 log for another use.
3. Preheat oven to 425°.
4. To prepare tenderloin, combine mustard and next 4 ingredients (through 1/2 teaspoon pepper); stir with a whisk. Spread mustard mixture over all sides of tenderloin; sprinkle with 2 tablespoons thyme. Place tenderloin in a shallow roasting pan coated with cooking spray. Bake at 425° for 38 minutes or until a thermometer inserted in center of tenderloin registers 135° or until desired degree of doneness. Let stand 10 minutes.
5. Cut tenderloin crosswise into 16 slices. Arrange 2 slices on each of 8 plates. Cut 1 butter log into 8 slices; top each serving with 1 butter slice.