Pat Beef Tenderloin Steak (2) dry with a paper towel and season generously with Kosher Salt and Freshly Ground Black Pepper (to taste) on both sides.
Add Oil (1 tablespoon) to a medium stainless steel skillet and heat on high. When the oil is hot add steaks and cook 4 to 4 1/2 minutes per side for medium-rare, flipping only once and getting a good sear on both sides.
Remove steaks to a plate and cover with foil.
Add Balsamic Vinegar (1/4 cup), Beef Broth (1/3 cup) and Fresh Rosemary (1 sprig) to the same pan whisking to pick up any beefy bits left behind by the steak (deglazing). Continue to cook over medium-high heat for about 4 minutes until sauce is reduced by at least half and can coat the back of a spoon. Remove rosemary sprig.
Turn heat to low and whisk in Unsalted Butter (1.5 ounce) one chunk at a time, allowing the butter to be incorporated before adding more. Once all the butter is incorporated remove from heat and adjust seasoning to taste with Kosher Salt and Freshly Ground Black Pepper (to taste). Serve over steak or on the side as a dipping sauce. Makes about 1/3 cup of sauce.