Put the white sugar in a large bowl and add the egg yolks, and then whisk briskly until the mixture lightens in color.
Reserve 1/4 cup of cold milk in a bowl and slowly add the cornstarch and whisk until there are no lumps. Leave this to one side.
Pour the rest of the milk into a saucepan and gently heat, along with the stick of cinnamon and the orange zest.
When the warm milk reaches boiling point, lower the temperature to a simmer. Remove the cinnamon stick and orange zest then temper the egg-sugar mixture into the milk.
Allow the mixture to warm through again and then add the milk-cornstarch mixture -- keep stirring and remove the cinnamon stick. Continue to simmer the mixture until it starts to thicken to a creamy thick custard consistency, being careful not to let it burn or get lumpy.
When you've reached the desired consistency, pour the mix into the ramekins and allow it to chill in the refrigerator for at least 2 hours.
Just before you serve them, evenly sprinkle a generous amount of sugar onto each serving, and either place the ramekins under a hot grill or use a cook's blowtorch to gently caramelize the sugar.