Heat oven to 350 F. Coat 6 (6-ounce) custard cups with cooking spray. Separate eggs and place whites in a mixing bowl; place egg yolks in a separate mixing bowl. Using a stand mixer or electric mixer at high speed, beat egg whites and salt. Gradually add 1/4 cup of sugar; beat until sugar is completely dissolved and stiff peaks form. Using a whisk, beat egg yolks and 1/2 cup of sugar until well-blended; add milk, lemon juice, flour, lemon peel and butter. Mix until smooth, about 2 to 3 minutes. Using a rubber spatula, gently fold egg whites into egg yolk mixture until just combined. Place 1/2 cup of the mixture into each custard cup. Set custard cups in a 13-by-9-inch baking pan; place in the oven. Fill baking pan with boiling water until water reaches halfway up the sides of the custard cups. Bake for 40 to 45 minutes until pudding tops are golden and firm. Remove from oven and baking pan and cool custard cups on a wire rack.