Can it be done? I have this idea of a cake in my head and I was wondering if anyone could tell me if it could be done successfully? I would love to do a chocolate cake w/ white butter cream and then pour dark chocolate ganache over the top and do a brushed embroidery technique on the top. Would that work or will the ganache be to sticky?

It could work if you chill the cake well so the ganache is very firm before you begin the brushwork. If I were you I would do a test run either on a small cake or even just by pouring some ganache on a sheet pan and trying the technique there.