I first make a classic “ichiban dashi” 一番だし broth” using kelp and dried bonito flakes. For a large sac of cod roe or “tarako” たらこ, I used about 1/2 cup of hot (temperature) broth and dissolve 3/4 one oz package of unflavored gelatin powder (Knox powdered gelatin). If gelatin does not completely dissolve, you may want to microwave it for few seconds. Then I divide the gelatin-dashi solution into two 1/4 cup portions. One is used for the tarako (a) and the other for the yam (b).
(a) For the tarako, I add a few drops of tabasco, dark sesami oil and soy sauce in the gelatin-dashi solution Let it cool down to the room temperature before adding to the cod roe. Remove the roe from the sac using the back of a knife and add the gelatin solution to the roe just so that the mixture becomes loose (I do not use all the gelatin solution). If you add too much gelatin solution, the upper layer will be only flavored gelatin when it sets. I lined a small square loaf pan (disposable aluminum kind) and lined it with plastic wrap. Pour the tarako-gelatin mixture into the pan (It will be about 1/2 inch thick) and place it in the refrigerator for 5 minutes until it jells.
(b) For the yam, I add 1 tsp of seasoned sushi vinegar to the gelatin-dashi solution. Cut the yam to small dices (I use about the half to 1/3 volume of the tarako). Pour in the gelatin solution (room temperature) just enough to coat every pieces of the yam. Gently mix, paying attention not to make too many bubbles since they do not look good in the final dish. Make sure the bottom layers of tarako is set. I layer the yam-gelatin solution on the top. Place it in the refrigerator for 5-10 minutes until set.