If you use a crusting buttercream and have a very gentle touch, you could do it on buttercream, but ideally, fondant is the best medium on which to do brushed embroidery. 
I've had good results with the buttercream. I'm in the minority here and avoid fondant at all costs--I just hate the taste and texture of it, even the MMF. Anyway, yes, you can do the brushed embroidery on buttercream without difficulty. Practice on waxed paper first, it only takes a few attempts before you get the knack for it, it's a nice technique to know. Good luck!
Yes, royal icing. I've done it and it's really quite simple and pretty. I want to actually do a whole cake that way but haven't gotten around to it yet.
Can't say that i am sure what exactly "brushed embrodery" is. Please Help!
Here's some instructions for you.
Brush Techniques
1. Make a flower pattern using a gum paste flower cutter, then outline one area at a time with thin consistency buttercream icing.
2. Before each outline can dry, immediately brush out lines of icing towards center of pattern area using a dampened decorator brush. Work in short, quick strokes.
3. Clean brush in water after brushing each flower, to create distinct lines of icing. Gentle brushstrokes of icing, piped directly on your cake, set a soft, romantic tone for wedding and anniversary cakes. Be sure to brush the design immediately after piping your outlinesthe technique won't work on dry icing.
I did it on my Course 3 final cake and it turn out fine. I did it all in and on buttercream. I did not have the time to do fondant. I did re-trace the flower again. Christine
Hi Farrah -
It can be done on buttercream with buttercream. I think it's one of the most beautiful, elegant techniques. There is a cake by ellepal that's purple and white and really stunning.
Good luck and have fun!
Christy
Quote by @%username% on %date%
%body%