My log saw something last night.
This cake is a combination of three cakes: Smitten Kitchen‘s “Heavenly Chocolate Cake Roll” (with coffee) with the whipped filling, cherry-pie filling from “Black Forest Cherry Roll Cake” on Food.com, and the frosting from “Bûche de Noël with Marzipan Mushrooms” from Bon Appetite via Epicurious. All of the recipes worked fine, and I’m not sure I can explain them any better, so I’m not sure if there’s a point to rewriting them here.
I will say that for the roll cake, leaving it in the towel overnight (it was late and I was very tired) to cool is not advisable–mine broke, but at least it broke into several long pieces that I was able to reassemble into a slightly flat log. Use a tea towel as other towels will stick. SK unrolled hers after an hour; I’d venture two hours is optimal. I trimmed the edges of the cake to build up the branch on top. (It’s also hard to get a modified Bûche de Noël to not look like a turd.)
Notes: The base cake is gluten free–the base is chocolate and eggs; no flour necessary! For my additions of cherry filling and ganache, make sure you have gluten-free corn starch and powdered sugar.
Regarding making this in Japan, the pan you’ll need is probably too big for the moven, but I think you may be able to halve the recipe. Japan also does roll cakes, so you could try a roll cake (rôru kêki, ロールケーキ) recipe on Cookpad (日本語, English) as the base instead. Kirsch (kirushu, キルシュ ) is relatively easy to find in liquor- and import stores and is gluten free. For the chocolate, I’ve used chocolate bars with good luck, especially for this recipe, which doesn’t call for unsweetened chocolate.