Preheat oven to 325°. In a small bowl, beat butter and sugar until crumbly. Add flour, salt, lime zest and juice; mix well. Beat in milk and egg yolks until smooth. In another bowl, beat egg whites until stiff peaks form; gently fold into batter.
Place 6 ungreased 6-oz. custard cups in a baking pan large enough to hold them without touching. Pour egg mixture into cups. Place pan on oven rack; add very hot water to pan to within 1/2 in. of top of cups.
Bake, uncovered, until a knife inserted in the centers comes out clean and tops are golden, 40-45 minutes. Immediately remove cups from water bath to a wire rack. Serve warm or at room temperature.