from Cook's Illustrated:
Foolproof Chocolate Frosting
Makes about 3 cups, enough to frost one 9-inch 2-layer cake
This frosting may be made with milk, semisweet or bittersweet chocolate. Cool the chocolate to between 85 and 100 degrees before adding it to the butter mixture. The frosting can be made 3 hours in advance. For longer storage, refrigerate the frosting, covered, and let it stand at room temperature for 1 hour before using.
20 tablespoons (2-1/2 sticks) unsalted butter, softened (60 to 65 degrees)
1 cup (4 ounces) confectioner’s sugar
3/4 cup Dutch-processed cocoa
Pinch table salt
3/4 cup light corn syrup
1 teaspoon vanilla extract
8 ounces chocolate, melted and cooled slightly (see note)
In food processor, process butter, sugar, cocoa, and salt until smooth, about 30 seconds, scraping sides of bowl as needed. Add corn syrup and vanilla and process until just combined, 5 to 10 seconds. Scrape sides of bowl, then add chocolate and pulse until smooth and creamy, 10 to 15 seconds. Frosting can be used immediately or held (see note)
This is no good for staying up on the sides of a layer cake but it is heaven on a cake-in-the-pan that you pour it over (or brownies): Put in saucepan 1 cup sugar, 1/3 cup milk, and 1/3 cup butter. Bring to a boil and boil softly for 1 minute by the clock. Remove from burner. Dump in 1 cup chocolate chips and stir until melted. Add 1 tsp vanilla. Immediately, pour over warm or cooled cake. This frosting sets up as it cools and turns to a layer of fudge.
This should do it for you. Don't let the sour cream scare you - it's delicious and not at all sour-creamish. Very very smooth and thick.
Fudgy Sour Cream Frosting
2 cups semisweet chocolate chips
1-1/2 cups sour cream
1 tsp. vanilla
In a double boiler, or in a saucepan set over a pan of hot (not boiling) water, melt the chocolate chips, stirring until smooth. Transfer to a mixing bowl.
Add the sour cream and the vanilla and beat, with an electric mixer on medium speed, until smooth and creamy. Let cool just slightly - until the frosting begins to thicken. But don’t wait too long because it will quickly thicken beyond spreadability. Spread frosting evenly over cake or brownies, then chill until set.
Makes enough to fill and frost one 9-inch two-layer cake, or a couple of pans of brownies.
This was my go-to recipe after you posted it a while back. But, I forgot the proportions and tried googling it. I found this chocolate sour cream one at epicurious.com which is similar but a little creamier that uses a combination of milk chocolate and dark. I now play with proportions, depending on the type of cake it is and how rich of a chocolate flavor I want:
Another one I like is RLB's milk chocolate buttercream from the Cake Bible. Again, it uses a combination of milk and dark chocolate and I play around w/ proportions. It's just milk chocolate, dark chocolate, butter, vanilla. I can find the proportions if anyone is interested.