.btn {
  border: 0 solid;
  cursor: pointer;
  font-family: system-ui;
  font-size: 100%;
  background: linear-gradient(90deg, blue, red);
  border-radius: 999px;
  font-weight: 900;
  padding: 1.8rem 5rem;
  position: relative;
  text-transform: uppercase;
}

.btn span {
  background: #1e293b;
  border-radius: 999px;
  color: #fff;
  display: grid;
  inset: 5px;
  place-items: center;
  position: absolute;
  transition: background 0.3s;
}

.btn:hover span {
  background: none;
}