You are going to want to get out your stand mixer for this one, it'll make life much easier.
First, attach the paddle attachment to your mixer. Add the butter and beat on high until the butter is whipped. It'll have a fluffy appearance.
Next, add in the remaining ingredients except for the powder sugar. Mix until combined.
Add in 1/2c of the powdered sugar at a time on low speed. This does take a little while. You may need to add more heavy cream or powdered sugar to get the perfect consistency.
Once everything is combined, turn your mixer on high for 2-3 minutes. This will give you the light fluffy buttercream. Then turn it down and beat on low for about 30 seconds to get out any air bubbles.
You're done! It's that simple. I will say this denser than some of the other buttercreams I've made. I think it really has to do with the American Heritage Chocolate drink, but I still liked it.