Mix the milk, egg and cinnamon together in a bowl and beat well. Take two slices of thick cut bread and 'dunk' them on both sides in the milk/egg/cinnamon mixture. Heat the butter in a frying pan then place the bread in the pan and heat gently until the underside is browned. Turn the bread over and cook the other side in the same way. You may have to add more butter as you're cooking.