Melt butter in a heavy saucepan over low heat. Stir in cocoa. Gradually add condensed milk stirring constantly until smooth. Cook over medium heat stirring constantly until thickened and smooth (about 3 minutes). Remove from heat and stir in vanilla. Pour mixture into lightly greased 8 inch square pan. Cover and chill 3 hours or until firm.

Shape mixture into 1 1/2 inch balls. Roll in cocoa. Place balls in mini baking cups. Store airtight in the refrigerator up to 1 week.