How many calories are in buttercream?
Depends on the recipe, and the serving size!
Real buttercream has a lot of calories (at it's essence it's butter, sugar, and eggs.) There are internet sites with calorie calculators that you can enter your recipe in to find the answer.
A lot! A tablespoon of butter has about 100 calories. A tablespoon of refined sugar has about 50 calories. Frost a cake and do the math!