Place chocolate and extra butter in a small saucepan over low heat. Cook, stirring for 2-3 minutes or until melted and combined. Pour over chilled cheesecake. Spread with spoon. To get a flat surface, tap the pan several times on the bench top and shake pan a few times. Chill for 1 hour or until chocolate is set. To cut, use a knife heated in hot water. Wipe knife clean between each cut.