Heat 50g of the butter in a pan, add the vinegar and shallots and cook gently until soft. Cool slightly. Fold into the remaining butter, along with the chillies and mustard. Refrigerate until ready to cook the steaks.
Pan-fry the steaks for about 30 seconds each side (or until cooked as desired). Remove from the pan and rest. Add some chilli butter to the same pan, as little or as much as you like, and swirl it around until melted. Pour over the steak and garnish with watercress (optional).