Preheat oven to 350°F. In a medium bowl, beat together eggs, sugar and salt until well blended. Stir in milk, lemon peel, lemon juice and vanilla. Blend well. Place four (6 oz.) custard cups or ramekins in an 8 x 8-inch baking pan. Pour about ¼ egg mixture into each custard cup. Place pan on center oven rack and pour very hot water into pan so it comes within ½-inch of the top of the custard cups. Bake until a knife inserted in the center comes out clean, or about 30-35 minutes. Remove custard cups from hot water. Cool on wire rack about 5-10 minutes. Serve warm or cold. Refrigerate leftovers.