Preheat oven to 325°. Arrange shells on a baking sheet and defrost at room temperature, 15 minutes. Meanwhile, beat egg white until watery but not foamy.
With a fine-bristle pastry brush, generously brush inside of shells with egg white, gently working it into crevices. Bake until white is set, about 5 minutes.
In a large measuring cup (or other vessel with a spout), whisk yolks, sugar, and milk to combine. Skim off and discard any foam on top of the mixture and let sit 5 minutes (this will ensure tarts with sunny, shiny tops). Pour into shells and bake until set, 8 to 10 minutes.
Note: Nutritional analysis is per tart.