1 stick butter, no substitutions
1 1/2 cups white sugar
1 1/4 cup unsweetened cocoa powder
1/8 teaspoon salt
1 1/4 heavy whipping cream
1/4 cup sour cream
2 teaspoons vanilla
2 cups powdered sugar (you may need less; you may need more)
Combine the cream and sour cream in a measuring cup, mixing until smooth (or as good as you can get it). Set aside.
Place a large saucepan with the butter in it over medium heat to melt the butter. Over the heat, stir in the sugar, cocoa, and salt. The mixture is going to be grainy and thick.
Slowly add the cream mixture, while whisking. Continue to cook without boiling until the sugar is dissolved (about 6-8 minutes). If necessary, reduce the heat to medium low.
Remove from heat and stir in vanilla. Allow to cool at room temperature, then whisk in the powdered sugar until desired consistency. Use as desired.