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Desde el controlador al hacer la llamada al view, especificar layout:

return View("Index", "~/Views/Shared/_StaffLayout.cshtml", someViewModel);

@{
    Layout = "~/Views/Shared/_LoginLayout.cshtml";
    ViewBag.Title = "Login";
}

Layout por defecto en archivo _ViewStart.cshtml

@{
    Layout = "~/Views/Shared/_Layout.cshtml";
}