Smooth and creamy Milk Chocolate Frosting has a soft texture that doesn't harden and sets up like luscious soft fudge in the fridge. It is the milk chocolate version of my BEST Chocolate Frosting that I use for my popular BEST Moist Chocolate Cake.
Finely chop the milk chocolate or use these couverture milk chocolate chips and place it all in a heatproof bowl. If you are using these couverture chocolate chips then there's no need to chop them.
Pour cream into a 1-quart stainless steel saucepan over low medium-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. Since there is a lot more chocolate than cream, you may need to gently heat the mixture over a double boiler while stirring to melt all of the chocolate and make a smooth shiny ganache. To do this, add about 1 inch of water to a small saucepan and place over medium heat. When it reaches a simmer, rest the bowl with chocolate ganache over the pot to let the steam heat it from below.
Remove the bowl from over the heat. Sift in cocoa powder, then add sour cream and salt 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 with a spatula. 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 a few seconds using a wire 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. Use the frosting immediately. It will firm up as you spread it.
Use a large offset spatula to spread the frosting over cooled cake layers right away. I find that if you use light cream, this frosting is more fluid and forgiving. If you use heavy cream, it will stiffen up a bit as it cools down so try to avoid spreading around too much as the back and forth motion will cause the cocoa butter to crystallize quickly.