In a saucepan pour half of the milk, add butter and put on a small fire. At this time, throw in a bowl the egg yolks, the remaining milk, sugar, cocoa, starch and knead with a spoon or beat with a mixer. To the hot milk with the butter add the contents of the bowl. Over medium heat and, stirring all the time, bring to thick. When thickens (after the boil) for another minute to hold the gas to extinguish the fire and mix thoroughly until it is lump-free. Ready!!!