Buttercream icings are really adaptable because you can add whatever flavors of your choice. The key thing to remember is that the more fluid your flavoring, the more powdered sugar you must add to obtain that thick frosting like texture. So it is best to stick to extracts or more viscous additions, unless you simply adore sweetness to the max. Buttercreams consist of softened room temperature butter, powdered sugar, and flavoring. Also, sometimes milk/cream is needed to reduce the thickness of the frosting into order to achieve the desired texture. Other than that, that is basically it!
Want to make a crazy good chocolate buttercream? Replace one cup powdered sugar with one cup cocoa powder. Want to make a vanilla buttercream? Use vanilla extract. Want to make a nutella buttercream? Use nutella. Want to make [enter flavor here]? Use [enter flavor source here]. In all cases, use milk/cream/food coloring to thin it out if necessary. How much of it you use is up to you!
I'm going to demonstrate how to make a simple strawberry buttercream using strawberry jam. The reason why I don't use pureed strawberries is because it is too "liquidy" but if you prefer it, I suggest that you cook it down on the stovetop (which will also get you a more intense strawberry flavor). I feel that half the work is already done for me by using strawberry jam though.