<button class="gradient-btn">
  <span>SOME TEXT</span>
</button>

.gradient-btn {
  display: inline-block;
  position: relative;
  border: none;
  background: transparent;
  padding: 15px 30px;
}
/* Background */
.gradient-btn::before {
  content: '';
  display: block;
  position: absolute;
  z-index: 2;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  border-radius: 16px;
  background: linear-gradient(0deg, rgba(11,36,255,1) 0%, rgba(255,0,254,1) 100%);
}

/* Border */
.gradient-btn::after {
  content: '';
  display: block;
  position: absolute;
  z-index: 2;
  left: 2px;
  top: 2px;
  width: calc(100% - 4px);
  height: calc(100% - 4px);
  border-radius: 15px;
  background: linear-gradient(0deg, rgba(0,0,0,1) 0%, rgba(74,74,74,1) 100%);
}

.gradient-btn span {
  position: relative;
  display: inline-block;
  z-index: 3;
  color: #fff;
  font-weight: 700;
  font-size: 30px;
}