Unknown Lady by Angelo Bronzino. She could be <a href="//commons.wikimedia.org/wiki/Bianca_Cappello" class="mw-redirect" title="Bianca Cappello">Bianca Cappello</a>.
<bdi><a href="https://en.wikipedia.org/wiki/en:Bronzino" class="extiw" title="w:en:Bronzino"><span title="Italian Mannerist painter">Bronzino</span></a>
</bdi>
circa 1575 <div style="display: none;">date QS:P571,+1575-00-00T00:00:00Z/9,P1480,Q5727902</div>-85
National Gallery, London