JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">

<body>

<div class="container">
  <h2>Circle Button</h2>
  
  <button type="button" class="btn circle">1</button>
  
</div>

</body>

CSS

.circle {
  border-radius: 100%;
  box-shadow: 0 0 0 1pt black;
}