<button class="button-maximaze">□</button>

.button-maximaze {
  line-height: 34px;
  width: 34px;
  height: 34px;
  border: none;
  background: white;
  box-sizing: border-box;
  cursor: pointer;
  text-indent: -20px;
  font-size: 30px;
  font-family: serif;

  &::before {
    position: relative;
    content: '□';
    left: 22px;
    top: -4px;
  }
  &:hover {
    outline: 1px solid black;
  }
}