*It is important to use semisweet chocolate with 50-55% cocoa solids. If you use anything darker, the frosting will not be sweet enough and the consistency will be firmer. These semisweet dark chocolate chips have a high cocoa butter content and will make the best frosting. See tips above in FAQ section for how to make this frosting with other types of chocolate.
Finely chop the chocolate and place it in a heatproof bowl. If you are using these couverture chocolate chips then there's no need to chop them.
Combine whipping cream and honey or corn syrup in a small 1 quart saucepan over low heat and bring it to a gentle boil. Once it reaches a boil, immediately remove it from the heat and pour it over the chopped chocolate. Cover the bowl and let stand for 2 minutes. Remove the cover and stir in concentric circles using a spatula or a whisk until smooth and glossy.
Add sour cream, sift in cocoa powder and whisk it through gently. Do not whisk vigorously because incorporating air at this stage will cause the ganache to split.
Place the bowl in the fridge for about 10 minutes, then stir gently. Let chill for another 10 minutes or so until thickened. Do not leave it in much longer or it will set up firm. If it is still really runny, then place it back in the fridge for another 5 minutes.
Once it is thick like the consistency of pudding, whisk vigorously for about 15 seconds by hand using a wire whisk until thick and lightened in color just a bit. Stop whisking if it starts to look matte and thick - just whip a few times to incorporate some air but it should still stay glossy. You'll notice it will start to thicken as it sits as the chocolate sets around the fine air bubbles. Use the frosting immediately. It will continue to firm up as you spread it.
Use an offset spatula to spread the frosting over the cooled cake layers right away and avoid spreading around too much as the back and forth motion will cause the cocoa butter to crystallize and make the frosting stiffen up. *NOTE: This recipe makes just enough to cover a 2 layer 8-inch round cake with a thin layer to cover the sides because it is very rich. If you prefer a thicker layer of frosting, simple double the quantities of all ingredients and then it will take longer for the mixture to cool until thickened since there is a larger volume.