Fudgey Chocolate Truffles

Melt butter in heavy 2-quart saucepan over low heat. Add 3/4 cup cocoa; stir until smooth. Add sweetened condensed milk; stir constantly until mixture is thick, smooth and glossy, about 5 minutes. Remove from heat; stir in rum and finely chopped nuts, if desired. Pour mixture into flat pan. Refrigerate until firm, about 4 hours. Shape into 1 1/4-inch balls; roll in nuts or cocoa. Refrigerate until firm, about 2 hours. Store, covered, in refrigerator.