Adjust oven rack to lower-middle position and heat oven to 325º F. Set six custard cups in a 13-by-9-inch baking dish. Bring 1 quart water to a boil in a medium saucepan.
Bring the evaporated milk and sugar to a simmer in a small saucepan, stirring, until sugar dissolves. Meanwhile, whisk egg, yolk, and vanilla in a 1- quart Pyrex measuring cup. Gradually whisk milk mixture into eggs, then pour into custard cups. Set pan in oven; carefully pour hot water into pan halfway up sides of cups. Bake until custards are set, 20 to 25 minutes. Remove custards from baking dish and cool slightly. Cover with plastic wrap and refrigerate until chilled, about 2 hours. Serve. (Custards can be refrigerated for up to 5 days.)
Recipe courtesy of Pam Anderson, author of The Perfect Recipe for Losing Weight & Eating Great (2008).