Preheat oven to 325°. Place shells on a baking sheet and defrost at room temperature, 15 minutes. Meanwhile, beat egg white until watery but not foamy. With pastry brush, 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 (with a spout) whisk yolks, sugar, and milk. Skim off and discard any foam and let rest for 5 minutes. Pour into shells and bake for 8 to 10 minutes, or until set.