Let the cake cool enough to handle. If it is burnt from the top, you can grate some burnt crumbles with a grater or use a serrated knife to scrape the burnt layer of the cake. If it cake is burnt from the bottom, then turn it, and with a serrated bread knife try to remove the burnt layer. You need to run the sharp edge of a serrated knife over the bottom of the cake and remove the burnt crumbles from the cake until you have a brown unburnt layer visible.