1 For the gravy, heat the butter in a pan and stir in the flour. Cook, stirring, until lightly browned, then stir in the meat and vegetable stock. Bring to a vigorous boil, season with salt and pepper and add the Cognac. Keep warm. (Alternatively, pour off most of the fat from a roasting tin in which you have cooked a joint of meat, stir in the flour and continue as above. )
2 For the Yorkshire puddings, mix the flour and salt and sieve into a bowl. Make a well in the centre and add the egg and a little milk. Mix smoothly, then gradually add the rest of the milk or milk and water to produce a smooth batter. Leave to stand for about 30 minutes. Put a little fat into each of 12 small baking tins and put into the oven at 200°C for about 5 minutes. Take the tins out of the oven and divide the mixture evenly between them. Return to the oven (middle shelf) and bake at 200-210°C for about 10-15 minutes until the puddings are well risen.
3 Meanwhile, heat some fat in a roasting tin, add the potatoes and roast for 45 minutes to 1 hour, turning occasionally, until golden brown. Serve with the Yorkshire puddings and gravy, sprinkled with a little pepper.