Wait, wait, wait. You are a pastry chef and have never used a stencil

? Some of the first 'designs' i did as an apprentice was simply a stencil, and then I sprinkled cocoa or powdered sugar to make an impressive, professional looking design, even though I had no artistic talent.
Well: you have a totally white background. Just place the stencil and sprinkle cocoa powder? Try this practice on a sheet of white paper and show your client. Not dark enough? Contact your cocoa supplier: they can supply, at substantial more $$$, a cocoa powder that is totally black (sorry, I have used it before, but the maker and product name escapes me at the moment). Also, use and charge your customer, for a can of food grade lacquer (again, sorry, manufacturer and product name escapes me).