i need to do a 2d lion head on a sheet cake. it's going to be iced in buttercream. I usually have no problem just going with the flow, but this has me stumped. Should I draw it in colored buttercream or make a fondant 'plaque'? one problem I have with that is that I have no brown dust, but I do have brown color i could paint with.

you could make a stencil, and then airbrush it, I have done that before,  if you don't have an airbrush you could buy a can of spray, from Michaels or your local cake shop.