The cake and the frosting both contain a "burnt sugar liquid" which is basically a caramel. You make it by dry burning sugar, adding heavy cream, and then adding coconut milk and lemon juice. Because you add so much cream and coconut milk to the burnt sugar, the liquid ends up a light tan color.
To make the cake, you cream together butter and sugar, add eggs and vanilla, and then alternately add in dry ingredients (flour, baking powder, baking soda, salt) and half of the burnt sugar liquid. You pour the batter into a well-greased Bundt pan and bake.
I had some problems with the frosting. The frosting seems like it should be simple -- just put the ingredients (butter, powdered sugar, rum, and the remaining burnt sugar liquid) in a food processor and pulse until the frosting is shiny and smooth. The first time I made the cake, I used room temperature butter, even though the recipe didn't specify any particular temperature for the butter -- after all, frosting is usually made with softened or room temperature butter. But I found that the resulting frosting was grainy and thin. I tried adding more powdered sugar, chilling the frosting, reprocessing it, gently heating it over a double boiler... nothing I did could eliminate the grainy broken texture. Because the frosting was so thin, it was more of a glaze than a frosting, and I could only get a thin layer on the cake because the excess simply ran off the sides (or pooled in the hole in the middle of the cake). I still served the cake, but was determined to make it again to see if I could fix the frosting.
The following evening I made the cake again, using cold butter for the frosting. The frosting was not grainy but it ended up with small lumps of butter that I had to strain out with a sieve. It was still too thin to hold its shape, so I chilled the frosting for about an hour and it ended up thick enough that I could spread it all on the cake and have it stay put. After adding some caramel shard topping garnish (sugar and a little bit of water, heated until melted and golden, poured out on a Silpat and broken into pieces), I got the cake pictured below, which was reasonably close to the cookbook photo.
The funny thing is that while the caramel shards do make for an exquisite presentation, the cake itself is so spectacular that it needs no extra bells or whistles. But there's nothing wrong with a little eye candy!
Recipe: "Burnt Sugar Bundt Cake with Caramel Rum Frosting" from Baked Explorations: Classic American Desserts Reinvented, by Matt Lewis and Renato Poliafito, recipe available here.