Step 1: In a small saucepan over low heat melt butter, add cocoa, stirring constantly until smooth and slightly thickened. Remove from heat; set aside to cool slightly.
Step 2: In large mixing bowl combine confectioners' sugar and milk; beat until smooth.
Step 3: Add chocolate mixture and vanilla. Beat on medium speed until smooth and slightly thickened, about 5 to 10 minutes.
Step 4: Cool at room temperature to spreading consistency, about 1/2 hour.