I've come up with another short-cut way to an impressive cake. Although it almost seems a shame to cover the swirls of the mixed yellow/chocolate cake, if ever there was a topping fit to do so, it would be this cold-set ganache.
As far as I can tell, ganache recipes are fairly standard. This is mine, tailored to the amount necessary for a double layer cake (with a layer of ganache within to hold the two pieces together). This is done simply because I do not have a lovely springform pan. I would encourage you to bake the cake this way if you are lucky enough to have a springform pan, as to not interrupt the swirls of the cake. Additionally, it will make topping the cake easier.
Side note: Why is Firefox telling me ganache is not a word?! Clearly Firefox has never tasted my friend Cassady's truffles.
For the cake you will need:
1 box chocolate/yellow swirl cake mix (and its ingredient requirements)
For the ganache you will need:
315g (approx 11 oz) dark chocolate, finely chopped
295ml (approx 1 1/4 cups) double cream (heavy cream)
2. Allow cake to cool completely. If your kitchen is warm, it may be useful to chill the cake after it has reached room temperature.
3. Prepare the ganache by slowly bringing the double cream to a gentle boil. Remove from heat immediately (do not allow it to scorch or bubble over!) and pour over the chopped dark chocolate in a medium bowl.
4. Set cake on a wire rack over a large backing tray, this will allow excess ganache to drip off when preparing.
5. Allow ganache to cool slightly before spreading a thin layer over the cake to lock-in any stray crumbs. If making a double-layer cake, spread first on the top of the bottom layer, place second layer on and cover all surfaces with thin layer.
6. Allow this thin layer of ganache to set (place in fridge to speed up process) before pouring the remaining ganache over the cake.
7. If ganache has started to thicken, use a flat knife/spatuala to drag the ganache from the center to the side of the cake, allowing it to drip down the sides.
8. I prefer to allow the ganache to set in the fridge before serving. There is something about cold dark chocolate that I adore, though this quick tempterature change may affect the way the ganache sets so if you're looking for a glisteningly smooth ganache, skip this step.