Cut up the butter and break the chocolate

Now melt the butter and chocolate in Bain Marie until a smooth

Next separate the egg yolk from the white's and put in to separate cup

Whisk the egg yolks and sugar until the mix is white(ish) and bubbles when you stop mixing

Now combined the egg yolks and the melted chocolate

Using very clean and dry tools, whisk the egg whites until the are very fluffy and stiff

Finally mix the chocolate and the egg white's carefully then you will have a thick mixture

Lastly put the mousse in the fridge for a few hours and it will be ready