In a bowl, mash the butter, parsley, garlic and Cognac; season with salt and pepper.

In a large skillet, heat the 3 tablespoons of olive oil. Add the leeks and season with salt. Cover and cook over moderately low heat, stirring occasionally, until tender, 10 minutes.

Light a grill or heat a grill pan. Brush the steaks with olive oil and season with salt and pepper. Grill over high heat, turning once, until nicely charred outside and medium-rare within, 5 to 6 minutes per side. Transfer the steaks to a carving board and let rest for 5 minutes. Slice the steaks across the grain and spread the butter all over the meat, letting it melt in. Serve with the leeks.