Stir together milk and zest in a saucepan; bring to a boil. Remove from heat, cover, and let stand 30 minutes. Reheat slowly.
Place rack in center of oven, and preheat to 325°. Line roasting pan with 2 layers of paper towels. Place 4 (6-ounce) custard or coffee cups in pan.
Whisk together eggs and sugar until well blended. Whisk 1/4 cup hot milk slowly into egg mixture. Continue to whisk, and strain in remaining milk; skim foam from surface. Stir in lemon extract, if desired.
Pour custard into cups. Put pan on oven rack, and fill pan with hot water halfway up side of cups. Bake at 325° for 40 to 45 minutes or until set. Transfer cups to wire rack, and let cool to room temperature. Cover and chill 2 hours or up to 2 days.