Place the sugar in a heavy-bottomed saucepan and caramelize on high heat stirring constantly. It will turn amber brown in about 5 minutes. When it is a uniform caramel color, add the butter, and stir it in. Once mostly combined, add the chocolate, and stir to blend. Pour it out onto a silpat or parchment lined sheet pan, and spread it with an offset spatula 1/8-inch thick. Let cool and break into small pieces. Store in an airtight container.