JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">           
</div>

CSS

.box {
    position: relative;
    z-index: 1;
    width: 10em;
    height: 10em;
    background: #cccccc;    
    margin: 10vh auto;
    border-bottom-right-radius: 16px;
}

.box::before {
  content: '';
  position: absolute;  
  z-index: 0;
  width: 40px; height: 40px;
  border-right: 1px solid #666666;
  border-bottom: 1px solid #666666;
  border-bottom-right-radius: 20px;
  bottom: -6px; right: -6px;
}

.box::after {
  content: '';
  position: absolute;  
  z-index: 0;
  width: 40px; height: 40px;
  border-right: 1px solid #999999;
  border-bottom: 1px solid #999999;
  border-bottom-right-radius: 24px;
  bottom: -12px; right: -12px;
}