カーソルホバー時、ボタン:hoverで背景色が変化するCSSのみで表現【HTML・CSS】

<div class="container">
<p class="center">ボタン:hover中心から背景色流れる</p>
<a href="" class="btn"><span>MORE</span></a>
</div>

.container {margin: 100px 0;}
a { margin: 0; padding: 0; font-size: 100%; color: #3b5d82; font-weight: bold; vertical-align: baseline; background: transparent; text-decoration: none; transition: opacity .4s ease, color .4s ease;}
.center {text-align:center;}
/* ボタン基本設定 */
.btn {
  position: relative;
  display: block;
  margin: 0 auto;
  border: 2px solid #3b5d82;
  border-radius: 2rem;
  overflow: hidden;
  width: 17rem;
}
.btn span {
  position: relative;
  display: block;
  text-align: center;
  line-height: 1;
  letter-spacing: .2em;
  text-indent: .2em;
  padding: 1.2rem 0;
  transition: .2s ease .2s;
}
.btn:hover {
  opacity: 1;
}
.btn:hover span {
  color: #fff;
}
.btn:before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background: #3b5d82;
  transition: transform .5s ease-in-out;
  transform: scale(0,1);
}
.btn:hover:before {
  transform: scale(1,1);
}