JSFiddle - React, Tailwind, and code Playground

HTML

<div class="box">
    <div id="ne"  class="corner"></div>
    <div id="nw" class="corner"></div>
    <div id="se" class="corner"></div>
    <div id="sw" class="corner"></div>
    <div class="box2"></div>
</div>

CSS

body {
    background: #D8D8D8;
}
.corner {
background:white;
height:20px;
width:20px;    
position:absolute;
}
#sw {
left: -2px;
bottom: -2px;
border-radius: 0px 20px 0px 0px;
border-top: 2px solid white;
border-right: 2px solid white;
background:#D8D8D8;
}
#se {
right: -2px;
bottom: -2px;
border-radius: 20px 0px 0px 0px;
border-top: 2px solid white;
border-left: 2px solid white;
background:#D8D8D8;
}
#nw {
left: -2px;
top: -2px;
border-radius: 0px 0px 20px 0px;
border-bottom: 2px solid white;
border-right: 2px solid white;
background:#D8D8D8;
}
#ne {
right: -2px;
top: -2px;
border-radius: 0px 0px 0px 20px;
border-bottom: 2px solid white;
border-left: 2px solid white;
background:#D8D8D8;
}

.box {
    position:relative;
    height:200px;
    width:200px;
    border: solid 2px white;
    background:#D8D8D8;
    border-radius: 5px -5px 5px 5px;
}
.box2 {
    position:relative;
    height:160px;
    width:160px;
    overflow:hidden;
    left: 20px;
    top: 20px;
}
.box2:before{
    content:'';
    position:absolute;
    left:0;
    margin:-20px;
    height:40px;
    width:40px;
    border-radius:100%;
    background:#D8D8D8;
    box-shadow:160px 0 0 #D8D8D8,
    0 160px 0 #D8D8D8,
    160px 160px 0 #D8D8D8,
    0 0 0 500px white;   
}