Melt the butter in a pan. Transfer to a large bowl and add chopped chocolate. Mix until the chocolate is melted and a uniform mixture
then add the cocoa powder, icing sugar. Beat on medium speed until it all comes together and is a little thick.
Then add the sour cream and beat well to form a consistency as per your desire
Use icing sugar to adjust the frosting consistency to be thicker
Use a little extra sour cream and whip well to make it a little fluffy to be able pipe into some beautiful rosettes. ( in that case the colour of your frosting will be a little lighter but it will taste heavenly)
This Frosting is good enough to fill and crumb coat, a 3 layer 8inch cake
This frosting can be stored in the refrigerator in an air tight container. and stays good for a week.