1. Melt butter in heavy saucepan over low heat. Add cocoa; stir until smooth. Add sweetened condensed milk; increase heat to medium. Cook, stirring constantly, about 4 minutes or until mixture has pudding-like consistency and is smooth and glossy.
2. Remove from heat; stir in vanilla. Cover; refrigerate 3 to 4 hours or until firm. Shape into 1-1/4-inch balls; roll in cocoa or powdered sugar. Refrigerate until firm, 1 to 2 hours. Store, covered, in refrigerator. About 2-1/2 dozen candies.
NUT TRUFFLES: Add 3/4 cup coarsely chopped toasted pecans to chocolate mixture when adding vanilla. (To toast pecans: Heat oven to 375°F. Spread 3/4 cup pecan halves or pieces in single layer in ungreased shallow baking pan. Bake 5 to 7 minutes, stirring occasionally. Cool before chopping.)
RUM NUT TRUFFLES: Decrease vanilla to 1 teaspoon. Stir in 2 to 3 tablespoons rum OR 1 teaspoon rum extract and nuts.
ESPRESSO TRUFFLES: Decrease vanilla to 1 teaspoon. Stir in 1-1/4 teaspoons powdered instant espresso or instant coffee when adding vanilla. Roll balls in cocoa or chopped nuts.
NUT-COATED TRUFFLES: Roll balls in chopped nuts.