Preheat oven to 500 degrees. Season beef with salt and pepper, then place in large roasting pan and sear all sides of roast on high heat until brown. Place roast in oven and cook for 30 minutes.
Remove meat from oven; reduce oven temperature to 350.
Place shallots around beef and coat top of roast completely with cocoa powder; cook until an instant-read thermometer inserted into thickest part of beef registers 130 degrees (for medium-rare), about 40 more minutes. Transfer beef and shallots to platter and cover with foil.
Add wine to roast pan; cook over medium heat until syrupy, scraping bottom of pan, about 3 minutes. Add beef broth and mustard; cook until reduced by half, about another 5 minutes.
Strain into a small bowl and add any accumulated juices from beef. Season with salt and pepper; transfer au jus to a gravy bowl and serve with beef and shallots.