In the bowl of an electric mixer fitted with the flat beater, beat the butter on medium speed until smooth and creamy, 1 minute. Add the sugar in a slow, steady stream and beat until the mixture is light and fluffy, 4 to 5 minutes; stop the mixer occasionally and scrape down the sides of the bowl. Add the eggs 1 at a time, beating well after each addition. Continue beating until fluffy, white and increased in volume, 3 to 4 minutes; stop the mixer occasionally to scrape down the sides of the bowl. Beat in the vanilla. Reduce the speed to low and add the flour mixture in four additions, alternating with the milk and beginning and ending with the flour, beating until smooth after each addition; stop the mixer occasionally and scrape down the sides of the bowl.