This is the recipe for Chocolate Buttercream, but of course buttercream can have other tastes too. You can try for example strawberry buttercream with some strawberry jam. Just try things out! :)
Grab the things you need. For chocolate buttercream you need chocolate, of course. It doesn't really matter what kind of chocolate you use, as long as it's enough. It depends on the kind of chocolate how much you need. When you have pure chocolate, you'd want to use less chocolate as you would if it'd be white. Just get what you think is enough, and just try it out. :)
For this recipe I used a whole bar of dark chocolate. (No special brand, the cheapest I found.)
Start melting your chocolate, you can find a how to on the link I put on the top of this step. While you are melting the chocolate you have enough time to do the next steps already :)
Make sure the butter itself isn't too cold. Let it get to room temperature. Get your mixer with these things and a bowl.
Cut the butter in blocks and put all the butter in the bowl. Add some of the powdered sugar, mix it, and add some more again, untill all the powdered sugar is in the bowl. When it's all mixed, mix for about one and a half minute on a high speed. Watch your chocolate meanwhile.
This is what the buttercream should look like. When the chocolate is all melted, let it cool down for a bit, but don't let it get hard. Get a little bit of the buttercream out and try to mix it and see if the butter in the buttercream melts. If the butter doesn't melt, it's safe to mix the chocolate with the buttercream.
Now you can use it for a pie, or cupcakes, or whatever you wanted it for!