Preheat your oven to 350°F (175°C). This is essential for ensuring that your brownies bake evenly and have that rich, fudgy texture.

In a saucepan over medium heat, combine cranberries, balsamic vinegar, and brown sugar. Stir until the mixture thickens slightly and cranberries begin to burst, about 5–7 minutes. Set aside to cool.

Season the tenderloin generously with salt and pepper. Heat olive oil in a skillet over medium-high heat and sear the beef for 3–4 minutes on all sides until browned.

Transfer the beef to a roasting pan and brush it with half of the cranberry-balsamic mixture. Roast in the preheated oven for 25–30 minutes or until a meat thermometer reads 130°F (54°C) for medium-rare. Let it rest for 10 minutes before slicing.

While the beef is resting, prepare the brownie batter. In a mixing bowl, combine melted butter, sugar, and cocoa powder. Then add eggs, one at a time, beating well after each addition. Finally, stir in flour and vanilla until just combined.

Pour half of the brownie batter into a greased 9x9-inch baking pan. Layer the sliced roast beef over the batter, followed by the remaining cranberry-balsamic mixture. Pour the remaining brownie batter on top, covering the beef completely.

Bake for 30-35 minutes, or until a toothpick comes out with a few moist crumbs. Let it cool in the pan on a wire rack before cutting into petit fours.