Also try this Chocolate Fudge Frosting that makes a slightly larger batch and has a slightly softer texture.
Instructions
Finely chop the chocolate and place it in a heatproof bowl.
Combine cream and honey or corn syrup in a small saucepan over low heat 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. Then, stir in concentric circles until smooth and glossy.
Sift in cocoa powder, add sour cream and whisk it through gently. Do not beat because incorporating air will cause the ganache to split.
Place the bowl in the fridge for about 20 minutes 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 thick like warm pudding, beat with a whisk until thick and lightened a bit. Stop whisking if it starts to look matte and thick - just whip a few times but it should still stay glossy. It will firm up as you spread it.
Use the frosting immediately. Spread it over cooled cake layers right away and avoid spreading around too much which will make it stiffen up.
*NOTE: This recipe makes just enough to cover an 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.