In the bowl of an electric mixer fitted with the paddle attachment, combine butter, 4 cups sugar, milk, and vanilla. Beat on medium speed until smooth and creamy, 3 to 5 minutes. Gradually add remaining sugar, 1 cup at a time, beating for about 2 minutes after each addition, until icing reaches desired consistency; you may not need to add all the sugar. Store in an airtight container for up to 3 days.