I have painted on fondant before, but not buttercream....my question is do you do it the same way? Lemon extract and gel? Do you let the buttercream harden a bit first?
Put it in the fridge and let it get very cold then paint with some kind of alcohol.
Yep, same way. Just let the BC crust first, or as Daisy1 says put it in the fridge to set it more quickly. You have to be a bit gentler with the brush than you do for fondant but otherwise it works the same way.
I personally would avoid the fridge, but in my area I would have to deal with condensation.
I have painted on the top of crusting buttercream. I just let it crust over then used soft brush and a light touch. My favorite thing to paint with is airbrush colors.
Check out my cake pics, the life size skateboard was buttercream what was painted on.