Edit in JSFiddle

ol {
    list-style-type: none;
    counter-reset: pizza;
    padding: 0;
}
li {
    padding-left : 25px;
    position:relative;
    margin-bottom: 5px;
}
li:before{
    content: counter(pizza);
    counter-increment: pizza;
    position: absolute;
    left: 0;
    right:0;
    background-color: blue;
    width: 20px;
    text-align: center;
    color: #fff;
}

<h2>Pizza na frigideira</h2>
<ol>
    <li>2 1/2 xícaras de farinha de trigo</li>
    <li>1 colher de manteiga</li>
    <li>0 colher pequena de sal</li>
    <li>1 copo de leite morno</li>
</ol>