Put the yeast in a small bowl and add a little lukewarm milk. Wait for it to bubble a little. Melt the butter and add the milk. Add the rest of the ingredients and knead for about 10-15 minutes. The easiest way to do this is in a food processor. Leave the dough to rise for at least 30 minutes.
Roll out the dough to about ½ cm (0.2 inch) thick and 30 by 15 cm wide (12 by 6 inch). Spread with butter and sprinkle with sugar and cinnamon.
Roll up the long side of the dough and cut into thick slices. Place the slices on a baking tray, cut side up, and leave to rise again for about an hour or until doubled in size.
Whisk the egg and water together. Pour the mixture over the buns and sprinkle with the caster sugar. Bake in the oven at 200˚C (400˚F) for 15-20 minutes or until cooked and golden brown.