Sift the powdered sugar to lighten and make it lump-free.
Place the softened butter in the bowl of a stand mixer fitted with a paddle attachment and beat about 30 seconds until smooth (longer for colder butter, shorter for softer. If the butter is very soft, just put the whole stick in without bothering to cut it first).
Slowly begin adding powdered sugar, mixing on low speed (to maintain some appearance of cleanliness in your kitchen), in stages. Once the first addition is well combined, add the next. Remember to scrape down the sides of the bowl to keep everything mixed in evenly. The mixture will become lighter in colour (more white, less yellow) and thicker as you add powdered sugar. For thicker frostings, you will want a higher ratio of sugar to butter, for thinner, keep them more even.
When all your powdered sugar has been added, beat the frosting on medium speed for 30 seconds.
You can now add the vanilla (if using) and some liquid (a little goes a long way!) to adjust the consistency to suit your purposes. If you add too much liquid, you can always add in more powdered sugar to stiffen it back up. This holds true at any time as well. If you’re frosting your cake and decide it’s too thin, mix in some more powdered sugar. If it’s too thick, add some more liquid.
Beat the frosting on a higher speed to make it more fluffy, about 1 to 2 minutes.
While this is mixing, melt your dark chocolate however you prefer, I use the microwave in 45 second increments. Mix the chocolate until smooth, and set aside.
Once it is at the consistency you want, pour in the melted chocolate and mix on medium speed until combined. Once fully combined, go back to high speed for another 30 seconds or so for some extra aeration.
You have frosting! Fill, crumb coat, and frost away! Or, keep the frosting very thick, scoop and roll into small balls, and freeze for 15 minutes then dip in tempered chocolate, to make some dangerous Dark Chocolate Truffles. Or be like me and take this opportunity to lick the bowl and beaters as a reward for your hard work!
Softened butter works best. You can use colder butter, but it will take much longer to get the nice smooth consistency you want and you may end up with flecks of unmixed butter in your frosting.Sifted powdered sugar is better, but to be honest I never sift mine and it works just fine.For the dark chocolate, chips, bars, or anything else works since you’re going to melt it. Also, the darker the better! Even for those who don’t like super dark chocolate, I’d still use dark chocolate if you have it because there is so much sugar in the frosting itself, it will still be very sweet and the darker chocolate has a richer flavor.Now normally I put vanilla in everything and usually about 3 or 4 times the recommended amount because I love vanilla extract. However, lately I haven’t been using vanilla in my buttercream and I’m finding it comes out just as good if not better, so the choice is yours.Store your frosting in an airtight container in the fridge. If you’re planning to use it again, let it soften on the counter for several hours before using (you can stick it in the microwave for 10-15 seconds to help soften it if you’re short on time). It’s even better if you can whip it on high speed in your mixer for 30 seconds for a better consistency before you use it (this of course assumes that you have any left over and didn’t eat it all before needing to use it again, which is something I wouldn’t understand).This recipe should be enough to frost and fill a 3-4 layer 6” round cake or any standard 8-9” round cake. I tend to use thick coats of frosting so I usually use more than expected.Remember, this is a basic outline and you can experiment as you see fit to make more or less frosting or to change the flavor or consistency to your taste. The amounts I gave are loose averages for what I generally end up using.