コード

<a class="btn btn01" href="">Button</a>

.btn {
	display: inline-block;
	width: 200px;
	height: 50px;
	line-height: 50px;
	text-align: center;
}
.btn01 {
	color: #fff;
	border: 1px solid #000;
	border-radius: 5em .5em 2.5em .5em/.5em 2.5em .5em 3em;
	background-color: #000;
	-webkit-transition: .3s;
	transition: .3s;
}
.btn01:hover {
	color: #000;
	background-color: #fff;
}