As promised - an update!
Here's the cake - I used one-third of this recipe. Cut into quarters, level and frost. A crumb coating is essential, as the cake is very moist.
For the Peanut Butter frosting, I used 1/2 the recipe to achieve a pretty thick coating. Perhaps if you're a better cake-froster than I, you could get away with less?
Not much waste!
This is what was left over using 1/2 the chocolate ganache. Next time I'd use 1/4 the recipe.