Whisk together cornstarch, mustard, sugar, salt, and 1/4 cup buttermilk in a small heavy saucepan until smooth. Whisk in remaining 3/4 cup buttermilk and egg until smooth. Cook over moderate heat, whisking constantly, until mixture reaches a simmer, then simmer, whisking constantly, until thickened and custardlike, 4 to 6 minutes. Remove from heat and whisk in lemon juice and oil. Transfer dressing to a bowl and chill, its surface covered with wax paper, about 1 hour.
