My daughter loves Minnie Mouse and this is the Minnie Mouse cake she wanted.
I used 2 9×13″ cakes and butt them up to each other. I drew a picture of Minnie on butcher wrap then I cut it out. I always ice my cakes with a crumb layer while they are frozen then I pop them back in the freezer to harden up the crumb layer so I can place the cut out design on the cake and then trace around it with a toothpick just to give me a nice clean outline of the design.
You can freehand the outline, but I like to do it this way just to be sure I am spacing it properly. Once you have the outline just pull away the design and go over it with your icing. The details are a lot easier to add in once you have your outline. I like to use different tips for different features.
I wanted Minnie’s ears to be less textured, but I did not want to use fondant so I just piped them in with little dots. For the rest I used the star tip.
I use the paste food coloring to get vibrant colors and I always make chocolate icing with cocoa before coloring with the black paste so I am not trying to turn white in to black.
I like your idea of making black icing out of chocolate icing rather than black out of white.