I actually found this
orange cream cupcake recipe and wound up basing it off that. I used a box mix of chocolate, substituted 1/2 cup water for 1/2 cup OJ (it called for 1 1/4 cup water, so I did the remaining 3/4 with straight water so it wouldn't get crazy), and added about 2 tbsp of fresh orange zest. For the frosting, I mixed OJ with cool whip and powdered sugar (to thicken it). The batter tasted subtly orangey, and so did the frosting. Right now, the cake is cooling!
I really wish I had one of those chocolate oranges like you pictured, though. That would be amazing to garnish each slice with.
Instead, I just have a bit of left-over zest (and one big piece for the center) and some basic cake decorating skills for piping frosting decorations.
If this turns out well, next I'd like to try chocolate lavender with a ganache frosting! My mom has a lavender plant and has agreed to hook me up with some stuff.