Moisture and temperature
The two most important things to consider are moisture and temperature. Butter does not mix well with (too liquid) moisture, so neither does buttercream. A small amount of liquid is still an option, but it is wise to add this in portions.
Temperature is also really important. Make sure your flavor ingredients are at the same temperature as your homemade buttercream. If the temperatures are too different you run the risk of your buttercream splitting. Same goes for adding too much liquid.
Chocolate Flavor
Probably the most popular buttercream flavor is chocolate. And with good reason as chocolate buttercream frosting is easy to make and can be done in two different ways. The easiest way is to add unsweetened cocoa powder. Just add the powder to taste and you have the best buttercream frosting in chocolate flavor.
Another way to add chocolate is to add melted chocolate to the cream. Make sure you let the chocolate get to room temperature (just like the cream) and then fold it into the cream. You can use the bowl of a stand mixer if you want and do this at low speed with the whisk attachment. Although doing this by hand is just as easy.
You can make white chocolate cream this way (so good! I’ll share my recipe soon!) Or use your favorite chocolate bar, the sky’s the limit!
Fruit Flavors
Using fruit to flavor your buttercream is also very good, but can sometimes be a bit tricky. You can put pieces of fruit through the cream, which is easy (and tasty!). But if you don’t want pieces of fruit, but a fruit flavor, you have to be a bit more careful.
You will be using fruit puree or fruit juice, which brings me back to the question of moisture. In order to avoid the risk of the mixture splitting you can boil down your (for instance) strawberry puree for making strawberry buttercream to reduce the amount of liquid in the puree.
This way you have evaporated most of the moisture and you’re left with a more concentrated flavor and a thicker liquid. You want the right consistency between the frosting recipe and the flavor you’re adding.