Prepare the custard. In a saucepan with a thick bottom, mix starch, sugar, eggs, add milk and put on a slow fire, stirring constantly to avoid the formation of lumps. As soon as the custard thickens and begins to boil, remove from the stove. Add soft butter and lemon juice to the hot custard, mix until completely dissolved. Cover the custard with cling film "in contact" and leave to cool at room temperature.