Peel potatoes and chop into small pieces of roughly the same size.
Boil potatoes until tender (approximately 15 to 20 minutes.) Drain and pour into a large mixing bowl.
Mash with butter, sugar, cinnamon, and milk while still warm. Allow to cool, then mix in one of the egg yolks.
Pour into a 9-inch round or square baking dish and use the back of a spoon to make a design of your choosing on the surface.
Break apart the remaining egg yolk with a fork and brush the entire yolk over the surface of the casserole being careful not to flatten the design. Sprinkle on sesame seeds if desired.
Bake for 10 minutes at 350 degrees to heat it through then put under the broiler for up to 10 minutes to brown the top.
