Place the soybeans in a bowl and cover them with water. Let them soak for at least 8 hours or overnight to soften.

Drain the soaked soybeans and transfer them to a pot. Cover with fresh water, bring to a boil, then reduce heat and simmer until tender, about 1 to 2 hours.

Once cooked, drain the soybeans again. Using a food processor or blender, pulse them until they form a smooth paste, adding a little water if necessary.

Transfer the blended soybean paste to a mixing bowl. Stir in the koji rice and sea salt until well combined.

Using a spatula, pack the miso mixture into a clean glass jar, pushing down firmly to eliminate air pockets.

Cover the jar with a clean cloth or lid and set it in a cool, dark place for 6 months to ferment.

After 6 months, check the paste for flavor and texture. It should be smooth, creamy, and have a deep umami flavor.

Once fermented, transfer the miso paste to a clean container and refrigerate. Use it as needed.