1. Stir together cocoa and 2/3 cup sweetened condensed milk in small saucepan. Cook over low heat, stirring constantly, until mixture is smooth and very thick. Remove from heat; stir in 1 teaspoon vanilla.
2. Beat 2 packages cream cheese in small bowl until fluffy; add cocoa mixture, beating until well blended. Cool thoroughly.
3. Beat remaining 2 packages cream cheese in medium bowl until fluffy. Gradually beat in remaining sweetened condensed milk, lemon juice and remaining 2 teaspoons vanilla. Alternately spoon vanilla and chocolate mixtures into prepared pie crust; gently swirl with knife or spatula for marbled effect. Refrigerate 8 hours or until firm. Garnish with chocolate curls, if desired. 6 to 8 servings.
CHOCOLATE CRUMB CRUST: Stir together 1-1/4 cups graham cracker crumbs, 1/4 cup HERSHEY'S Cocoa and 1/4 cup sugar in medium bowl. Stir in 6 tablespoons melted butter or margarine. Press mixture firmly on bottom and up sides of 9-inch pie plate; refrigerate about 2 hours before filling.