Heat the milk gently in a small pan until just warm. Put the yeast in a small bowl and add half the milk (ensuring that it is only lukewarm at the most) plus 1 tsp of the caster sugar. Mix well and leave for about 10 mins until foaming. In a large bowl (or in a stand mixer fitted with a dough hook) mix the flour, remaining sugar, a pinch of salt and the cinnamon. Once the yeast is foamy, add the remaining milk, egg and butter, and mix to bring together into a dough.