Preheat the oven to 350°F. Butter two cookie sheets. Sift the flour, cornstarch, and baking powder into a medium bowl. Beat the butter and confectioners’ sugar in a large bowl with an electric mixer at high speed until creamy. Add the egg yolk, lime zest and lime juice, and vanilla, beating until just blended. Mix in the dry ingredients. Beat the egg white and salt in a large bowl with an electric mixer at high speed until stiff peaks form. Use a large rubber spatula to fold the beaten white into the lime mixture.