Put the heavy cream in a saucepan, and set it over medium-high heat. As soon as it begins to simmer, remove the pot from the heat. Add the chocolate and stir with a wooden spoon to melt the chocolate. Stir in the corn syrup.
To cover a cake with poured ganache, set a wire rack in or over a baking tray. Set the cake on a rack. Carefully ladle the molten ganache over the cake in a steady stream, letting it run over the cake until uniformly covered.
Otherwise, transfer to a bowl and refrigerate for about 1 hour. If using for filling, soften in a double boiler over medium heat until pourable.