In a medium bowl, stir together the soy sauce, mirin, ginger, garlic and ¼ teaspoon each salt and pepper. Add the chicken and stir to coat. Cover and refrigerate for at least 2 hours or up to 24 hours.

Set a wire rack in a rimmed baking sheet. In a large bowl, whisk together the potato starch and ½ teaspoon each salt and pepper. Working a few pieces at a time, remove the chicken from the marinade, letting excess drip off, then dredge in the potato starch, pressing evenly to adhere on all sides. Shake off any excess and transfer to the rack. Let stand at room temperature while you heat the oil and make the sauce.

Pour enough oil into a 6- to 8-quart Dutch oven to reach a depth of about 1½ inches. Heat over medium-high to 350°F. Line a rimmed baking sheet with a doubled layer of paper towels and place near the stove.

While the oil heats, make the sauce. In a small bowl, whisk together the mayonnaise, lemon zest and juice, and shichimi togarashi (if using).

When the oil has come to temperature, carefully add half the chicken and fry, stirring to prevent sticking, until golden brown, 2 to 3 minutes. Using a mesh skimmer or slotted spoon, transfer the chicken to the paper towel–lined baking sheet. Allow the oil to return to 350°F, fry the remaining chicken and transfer to the baking sheet with the first batch.

Repeat the process to double-fry the first batch until deep golden brown, followed by the second batch. Transfer the chicken to a platter and serve warm or at room temperature with the dipping sauce.