In a small bowl, add the butter along with the finely chopped black garlic cloves, the porcini mushroom powder, the finely chopped fresh rosemary, and the salt and pepper.
Use a small spoon to throughly mix the ingredients together.
Spoon the butter mixture into the middle of a small sheet of plastic wrap, fold the plastic wrap over the butter, rolling gently to make the butter into a sausage-like shape. Twist the loose ends of the plastic wrap to seal the butter inside. Place the wrapped butter log into the fridge to harden for at least 1 hour.
Remove the chateaubriand from the fridge at least half an hour before cooking, to allow it to come to room temperature. Preheat the oven to 450 F.
Heat up a skillet pan to a high heat. Brush the chateaubriand with cooking oil and then place it into the pan. Sear for 5 to 6 minutes on each side, until a dark crust has formed.
Transfer the chateaubriand to a wire rack over a roasting tray, and then season well with salt and pepper. To cook the steak medium-rare, place into the oven and allow the chateaubriand to roast for 12 minutes, until the internal temperature reads 130 F to 135 F . If you prefer your meat less pink, allow the chateaubriand to roast a little longer.
Remove the chateaubriand from the oven and allow it to rest for at least 10 minutes, covered in foil. Carve and serve with wedges of the compound butter.