Add beaten egg, dashi stock, and soy sauce to a measuring cup and combine well.
Add flour into the cup and combine until the mixture is lump-free.
Cut the octopus into cubes about 1/2 inch.
Heat the takoyaki pan over medium-low heat. Grease each mold with oil. Extra oil will provide a crispier result.
Fill each mold with batter, trying not to overfill.
Place one piece of octopus into each mold along with tenkasu and shallots on top.
Pour batter over the mold and fill the surface of the pan.
Draw grid lines with a long bamboo skewer or a thin chopstick to portion each Takoyaki.
Scrape the edges of the batter and move everything toward the mold. Start with the most cooked pieces, using skewers or chopsticks.
Tuck excess batter into the mold while also turning the batter 90 degrees.
Turn into each mold again and turn another 90 degrees so the batter is completely turned over from its original position.
Cook balls for about 5 minutes. Turn frequently to allow them all to brown evenly.