Summarize:

You can give your buttercream a more interesting flavor by adding some extract, flavoring oil, or milk. Keep in mind that flavoring oils have a much more intense flavor than extracts, so you won't need as much. Here are some suggestions:  1 teaspoon of an extract, such as: almond, lemon, peppermint, or vanilla A few drops of a flavoring oil, such as: butterscotch, lemon, orange, or raspberry Simply add the ingredients to the powdered sugar and mix well. Here are some suggestions:  1 to 2 teaspoons of a spice, such as apple pie, cinnamon, or pumpkin pie, will give you a more aromatic buttercream, perfect for those fall and holiday treats. 1 tablespoon instant coffee mixed with 2 tablespoons of water will give you coffee-flavored buttercream. You can also add in a pinch of cocoa powder for a mocha-flavored buttercream. Substitute ½ cup (50 grams) of the sugar for cocoa powder. This will give you chocolate-flavored buttercream. Instead of using whipping cream, milk, or half-and-half, you can use 1 to 2 tablespoons of another liquid instead, such as fruit juice. Here are some more options:  Orange juice Lemon juice Strongly-brewed coffee Liquor, such as Bailey's, Kahlua , brandy, or rum Use 1 to 2 tablespoons of lemon or orange juice instead of whipping cream, milk, or half-and-half. Once you have finished making the buttercream, stir in ½ teaspoon of lemon or orange zest. Add up to 1/3 cup (110 grams) of your favorite jam into the butter and beat it until it is smooth. Mix in the powdered sugar and whipping cream, milk, or half-and-half as usual. Keep in mind that the jam will alter the color of the frosting. You can use any type of jam you want, but raspberry or strawberry jams will give you more traditional results.
Use extract or flavoring oil to add some flavor. Add some ground spices, instant coffee, or cocoa powder for extra flavor. Substitute the whipping cream for another liquid. Make a citrus-flavored buttercream. Use jams to flavor the buttercream. Finished.