JSFiddle - React, Tailwind, and code Playground

HTML

<div>29</div>

CSS

div {
   width:49px;
   height:49px; 
   border:1px solid black;
   position:relative;
   text-align:center;
   line-height:49px;
   font-size:25px;
}
div:before {
   content:'';
   position:absolute;
   width:0;
   height:0;   
   top:-1px;
   left:-1px;
   border:25px solid transparent;
   border-right:25px solid black;
   border-bottom:25px solid black;
   z-index:-3;
}
div:after {
   position:absolute;
   content:'';
   width:0;
   height:0;
   top:1px;
   left:1px;
   border:24px solid transparent;
   border-right:24px solid white;
   border-bottom:24px solid white;
   z-index:-2;
}