Place chocolate and heavy cream in a microwave safe bowl. Microwave until cream in very warm, but chocolate will not look melted. Stir mixture until the chocolate is melted. Add in corn syrup. Pour into a pastry bag or a zip plastic bag. Snip off the end. Start dripping chocolate one stream at at time down the sides of the cake, with some lines shorter than others until you pipe around the whole cake. Squeeze out chocolate on top of the cake and use a knife or off-set spatula to spread the chocolate on the top completely. Put remaining Nutella frosting in a pastry bag with a star tip or another design. Pipe frosting in dollops around the top of the cake.