Sieve the flour and salt together. Rub in the butter lightly and mix with the beaten egg and just enough milk or water to make a smooth dough. Roll out lightly one inch thick and cut in two-and-a-half inch rounds. Bake at 450°F, 230°C, gas mark 8 for ten minutes. Remove from the oven and split in half. Put on baking sheets with the cut sides upwards. Bake at 375°F, 190°C, gas mark 5 for about fifteen minutes until crisp and golden. Cool and serve with butter and cheese. These rusks store well in a tin.