JSFiddle - React, Tailwind, and code Playground

HTML

<button class="btn">
  Кнопка
</button>

CSS

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

.btn {
    position: relative;
    border: 2px solid blue;
    padding: 17px 67px;
    margin: 30px 30px;
    border-radius: 10px;
    background: #fff;
    font-weight: 600;
    font-size: 20px;
    cursor: pointer;
    transition: .5s;
    overflow: hidden;
    z-index: 10;
}

.btn:hover {
   color: #ffffff;
}

.btn:hover:before {
    width: 500px;
    height: 500px;
    left: 0;
    background: blue;
}

.btn:before {
    content: '';
    position: absolute;
    left: -25px;
    bottom: -30px;
    width: 30px;
    height: 30px;
    background: blue;
    transition: .8s ease;
    border-radius: 25px;
    z-index: -1;
}