Start by adding butter to the large mixing bowl and add 1-2 cups of icing sugar. Mix on low to start combining and then increase speed to medium-high and beat butter and sugar together for 2 mins until completely combined, mixture should be light in colour. This is the time to make sure your butter and sugar are completely creamed together or you may have small butter bits in your icing.
Once fully combined, scrape bowl with spatula to make sure no butter is sitting at the bottom. This is especially important with chocolate icing because any ingredients that are not completely combined are very noticeable.
Add remaining icing sugar. Be sure to start mixing slowly or all the icing sugar will come up and out of the bowl. Use a mixing bowl guard if you have one.
Once you start mixing the mixture will become very dry, add your vanilla and then milk. I usually add my milk half at a time mixing in between. Be sure to scrape the bottom of the bowl once or twice during this step to ensure everything is combined.
Add the cocoa powder and mix until combined. Again, scrape the sides and bottom of the bowl once or twice to ensure that it is fully combined.
Decorate accordingly!
Notes
Butter Temperature:If you are going to be piping with this icing, I recommend using cold butter and mixing longer on the first step until combined. This will give you the stability you need to pipe designs with.If you are frosting a cake then room temperature will be easier to work with as it will be easier to combine.