Combine buttermilk, sugar, and pineapple in a large mixing bowl, stirring well. Pour mixture into two 2-cup metal ice trays (with dividers removed); freeze several hours or until slushy. Remove from freezer.
Place mixture in a large mixing bowl. Add egg white and vanilla, beating well. Return mixture to ice trays; freeze until firm. Cut into squares or scoop out with ice cream scoop, and place in sherbet dishes.