How can I realise this special corner in my border with CSS3

This is what I got now:

http://jsfiddle.net/hashie5/nDv5k/

<aside>
<h2>Product in de kijker</h2>
<h3>Mobiele telefonie</h3>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
<a href="#" class="btn btn-runa">Ga verder</a>

    aside {
    border: 1px solid #CCCC00;
    border-radius: 10px;
    padding: 5px 20px 20px 20px;
    width: 290px;
    margin: 50px;
}
body {
    font-family:"Trebuchet MS";
    color: #333333;
    background-color: #FFFFFF;
    font-size: 14px;
    line-height: 150%;
}
h1 {
    font-size: 30px;
    color: #1F668C;
    line-height: 120%;
    font-weight: normal;
}
h2 {
    font-size: 22px;
    color: #CCCC00;
    line-height: 120%;
    font-weight: normal;
}
h3 {
    font-size: 22px;
    color: #1E678E;
    line-height: 120%;
    font-weight: normal;
}
h4 {
    font-size: 20px;
    color: #1E668C;
    line-height: 120%;
    font-weight: normal;
}
h5 {
    font-size: 14px;
    color: #CCCC00;
    line-height: 120%;
    font-weight: bold;
}
.btn-runa {
    background: none;
    background-color: #CCCC00;
    color: #FFFFFF;
    text-shadow: none;
}