Make dough: Place butter, sugar and salt in the bowl of an electric mixer; beat on high until mixture is creamy. Add the half-egg and beat 30 seconds. Add milk and beat on high speed 2 minutes. Add flour and beat on medium speed 5 seconds, then on high speed just until blended, about 5 seconds more. Over-mixing will produce a tough dough. Remove from bowl and shape into a 5-inch patty about 1/2 inch thick. Lightly dust patty with flour and wrap in plastic wrap. Refrigerate at least 1 hour, preferably overnight. (Dough will last up to one week, refrigerated).