In a classic spaghetti carbonara, eggs are whisked with a small amount of the water used to boil the pasta, which contains some residual starch and therefore has a slight thickening effect. The egg/water is stirred into the just-drained (but not rinsed) pasta that has been placed in the still-hot pan used to cook the guanciale (cured pork jowl), then the flame is turned off. The heat of the spaghetti and the pan gently cook the egg mixture, thickening it into a creamy sauce. Grated parmesan is mixed in, making the dish even creamier while also adding umami.
For this dish, I substituted sea urchin "tongues" for the eggs, and karasumi (salted and cured mullet roe; it's like bottarga) in place of the cheese, but the technique is almost the same as that for Italian carbonara. I serve this as a rich and decadent appetiser.
Set aside 6 nice pieces of uni - they will be used for garnishing the plates. Put the remaining uni into a bowl and whisk to break up the pieces. Continue to whisk until the uni is almost liquified (there will be a few tiny pieces).
Bring a pot of water to the boil, add the noodles and cook for one to two minutes, or until just tender. When the noodles are ready, turn off the flame. Ladle off about 90ml (¼ cup and 2tbsp) of the pasta water into a heatproof measuring cup. Pour 60ml (¼ cup) of the pasta water into the bowl with the uni and whisk immediately.
Drain the noodles then, without rinsing them, put them back into the hot pan they were boiled in. Add the uni/cooking liquid into the hot pan and immediately use tongs to mix and stir the ingredients. Add the butter and continue to mix until the sauce very lightly coats the noodles. If the sauce is too thick, add a little more of the cooking liquid. Divide the noodles between six plates and top each portion with some of the reserved uni. Grate some of the karasumi on top, garnish with shiso sprouts and shredded nori, then serve immediately.