Special equipment: four 10-ounce heatproof bowls or ramekins
Bring a pot of water to a boil.
In a large bowl, whisk together the eggs, chicken stock, fish sauce and salt. Pour the mixture into four 10-ounce heatproof bowls or ramekins. Arrange the bowls in a large, wide pot with a lid.
Add enough boiling water to the pot to reach two-thirds of the way up the sides of the bowls. Bring the water to a gentle simmer, cover the pot, and steam until the custards are slightly wobbly in the center, 6 to 7 minutes.
Divide the shrimp and scallions among the bowls, cover, and steam until the shrimp are cooked through and the custards are set, about 3 minutes more.
Top the custards with a drizzle of sesame oil and a sprinkle of sesame seeds. Serve warm.