In a large bowl, mix well Dashi, eggs, soy sauce, salt, Kimuchi juice and flour with a whisk.
Slice green onions finely. Cut Kimchi coarsely, and squeeze its juice out. Add both into the batter and mix well.
Cut octopus into 1/2" (1cm) pieces.
Mix mayo, Gochujang, and sesame oil in a small bowl, and set aside.
Heat a Takoyaki pan with oil to very hot, just until the oil begins to smoke. Use enough oil to coat the pan using a paper towel so that the batter won't stick. Then pour batter to fill the holes of the pan. Drop octopus pieces in the batter in each hole.
Cook at medium heat for 1-2 minutes and turn over using a Takoyaki turner (you can use a chopstick too). It can be a little tricky at first, so watch the video to see the technique. Cook another 3-4 minutes, turning constantly.
Place the cooked Takoyaki on a plate and pour on Takoyaki sauce and hot mayo sauce. Finish the dish by sprinkling the Takoyaki with cut roasted seaweed and sesame seeds.