Notes: It took about three times longer for the sugar to crystallise than in the video. I don't know why, but I suspect that the flour between the bowls is actually an absorbicant and I didn't realise that so didn't use as much. Crystallisation picked up markedly once I pulled off half the outer layer of aluminium foil and just covered with a towel instead.
Also, I used microwave tempering for the chocolate instead of the traditional method. The resulting tempering wasn’t bad! But I could’ve used more heat with the dark layer. I got the white chocolate up higher, and it set really well, so. I also severely underestimated the amount of white chocolate needed, which is why both layers (dark and white) are uneven thickness - I just didn't have enough to properly pour over all areas.
Overall, I'm pretty happy with it nonetheless.
Recipe can be found online in this YouTube video.
eta: It breaks up well! Popping from the chocolate side with a knife shatters it nicely. I expected that to be more difficult. The texture is really nice. Too sweet for sustained eating, of course, but for a bite or two there's just enough strength in the cocoa powder that combined with the surprising degree of crunch makes for a really nice experience.