Melt the butter in a saucepot over medium heat. Add the shallots and cook 1 minute. Add the flour and cocoa powder and cook 2 minutes. Whisk in the broth and wine and bring to a simmer. Whisk in the Worcestershire sauce, chocolate, cayenne and salt and pepper to taste and simmer until thickened, about 5 minutes, making sure to stir occasionally.
Heat the olive oil in a large skillet over medium-high heat. Sprinkle the beef with salt and pepper and cook until browned on all side but still medium-rare in the middle, about 6 minutes. Removes to a plate, cover with foil and let rest 5 minutes.
To serve, pour the sauce into a fondue pot. Arrange the beef, potatoes and cheese onto a platter and serve.