COMBINE 2 cups water, evaporated milk, sweetened condensed milk and Abuelita in large saucepan. Heat over medium heat, stirring occasionally, until mixture reaches a boil. Remove from heat.
COMBINE remaining 1 cup water, egg yolks and cornstarch in medium bowl. While stirring, slowly add some of the chocolate mixture to the egg yolk mixture. Continue to add small amounts of chocolate mixture until the egg yolk mixture is at the same temperature as the chocolate mixture. Add the egg yolk mixture back to the remaining chocolate mixture.
HEAT, stirring constantly, until mixture comes to a boil. Boil for 3 minutes. Remove from heat. Add vanilla extract. Strain, if desired. Serve hot.