Melt the chocolate over a double boiler (over hot water). You could also use the microwave — do take the container out every 10 seconds and stir, until it has melted.
Remove the chocolate from the heat, and while it is still warm add the sour cream.
Stir gently yet swiftly with a rubber/silicone spatula until the mixture is uniform in colour.
Ensure that the chocolate mass in your chocolate is not over 53% (curiously, PBS video that showcases Rose baking, she mentions that the chocolate mass should not be over 64%), as anything more makes your sour cream ganache bitter. This recipe yields 4 and 3/4 cups of indulgent sour cream ganache (765 g or 1 lb 10 oz) — enough to fill and frost two 9-inch by 1 and 1/2-inch layers or three 9-inch by 1-inch layers. You could also use a beater to beat the sour cream and chocolate together — you just end up with a lighter coloured sour cream ganache, since there's more air beaten into it. With a spatula, it'll be thicker and darker.