Preparation

Koji is the magical mold responsible for sake, miso, and soy sauce—basically, it's the grandfather of funky fermentation. When you cure egg yolks in a mixture of koji and salt, the enzymes break down proteins into pure umami while the yolks firm up into something between custard and cheese.

The result is intensely savory, slightly sweet, and endlessly versatile. Grate them over pasta, rice, salads, or anything that needs a hit of concentrated, fermented richness.

Preparation

1. Mix the koji rice (or powder) with the sea salt in a bowl until evenly combined.
2. Spread half of this mixture in a shallow container or baking dish.
3. Using the back of a spoon, create 6 small wells in the koji mixture, spacing them evenly.
4. Carefully place one egg yolk in each well, being careful not to break them.
5. Gently cover the yolks completely with the remaining koji mixture, ensuring each yolk is fully buried.
6. Cover the container with plastic wrap or a lid and refrigerate for 4-7 days. Check after 4 days—the yolks should be firm to the touch but still slightly yielding.
7. Carefully uncover and brush off the koji mixture from each yolk. Rinse them very gently under cold water and pat dry.
8. At this point, you can use them immediately, or for a firmer texture, place them on a wire rack in the refrigerator, uncovered, for an additional 2-3 days to dry out further.
9. Mix the toasted sesame seeds with nori flakes if using.
10. When ready to serve, grate or slice the cured yolks over your dish and top with the sesame mixture.

These cured yolks keep for up to 2 weeks wrapped in paper towel and stored in an airtight container in the refrigerator. They become firmer and more concentrated with time.