Blend the kombu and dried mushrooms together until they turn into a fine powder. It’s essential that the kombu and mushrooms are fully dry and contains no moisture at all. If they’re not fully dried, dry them in a dehydrator or an oven set at 70°C for at least 1 hour, or for up to 12 hours until they’re completely dry.

Sift the flour, salt, yeast, kombu and mushroom powder into a large bowl. Add the water to the flour mixture and mix with a wooden spoon or spatula until a sticky dough forms. Don’t knead it too much as it’ll cause the dough to be tough and chewy post-bake.

Pour half of the olive oil into a separate large container or bowl. (This container bowl should be ~3 times as large as the focaccia bowl.) Transfer the focaccia dough into the container, and pour the rest of olive oil around the sides of the dough, lifting the dough around the sides and letting the oil run down the sides. This helps the dough not stick too much to the container. Place the container in the refrigerator to proof for at least 8 hours, or for up to 2 days.

After the proof, the dough should be really poofy and pillow-y. Oil a baking tray (this recipe is meant for 18x13 baking trays, but it could work for smaller or larger ones too, though your focaccia will be slightly taller/flatter depending on the size of the tray.) Transfer the dough to the oiled tray, then using your fingers, spread the dough so it covers the whole tray. Add the oil from the proofing bowl as needed to prevent the dough from sticking to the tray. Leave the dough at room temperature for around 30-60 minutes, until it doubles in size.

Now using your fingertips, poke dimples on the surface of the focaccia dough and to get that characteristic crater-y focaccia crust. Bake the dough in an oven preheated to 220°C for 15-20 minutes, rotating halfway through the baking time, until the top is all a uniform golden brown.

When done, slide the focaccia off the baking tray onto a wire rack to cool down. It’s best eaten fresh out of the oven!