1. Make pudding: Whisk 1/2 cup milk, yolk and cornstarch until smooth. In a pan, stir 1 1/2 cups milk, salt and sugar. Bring to a boil over medium-high heat, stirring until sugar dissolves. Remove from heat. Whisking yolk mixture constantly, drizzle in hot milk.
2. Pour mixture back into saucepan and cook for 1 minute, stirring constantly as pudding thickens. Remove from heat; whisk in vanilla and butter. Pour pudding into a large bowl. Cover with plastic wrap so that plastic touches surface of pudding. Let cool for 20 minutes, then refrigerate until cold, at least 3 hours.
3. Chill 4 8-oz. glasses. Make gelatin: Put gelatin in a bowl. Pour in 1 cup of boiling water; whisk until gelatin dissolves, about 2 minutes. Stir in 1 cup of cold water. Keep at room temperature.
4. Assemble parfaits: Pour 1/4 cup gelatin into each glass. Chill until firm, 1 hour. Spoon in 1/4 cup pudding, smooth tops and freeze until firm, about 1 hour. Slowly pour 1/4 cup gelatin over pudding in each cup (you will use up gelatin mixture); chill until firm, 1 hour.
5. Top with berries and chill until ready to serve.