Mix first three ingredients and cook over moderate heat for 6 minutes, constantly stirring.
Lower heat and continue cooking for 10 minutes.
Add condensed milk and cook over low heat 20 minutes longer, stirring constantly.
Fill each coconut shell with mixture. Make a well in center and drop raw egg yolk.
Cover top with tuber mixture and spread until smooth, very close to brim or shell.
Cover whole shell with two layers banana leaves and tie securely with strings.
Steam half an hour 12 servings.