In a large mixing bowl, whisk together mochiko flour, potato starch, eggs, sugar, soy sauce, mirin, chopped scallions, minced garlic, and grated ginger until smooth and glossy.

Coat the chicken thighs in the marinade, cover and marinate in the refrigerator for at least 4 hours, preferably overnight.

Heat vegetable oil in a Dutch oven over medium-high heat until it reaches 350°F (175°C).

Carefully lower marinated chicken thighs into the hot oil, frying for 4-5 minutes on each side until golden brown.

Let the fried chicken cool on a wire rack for a few minutes, then drizzle with spicy mayo and sprinkle with furikake before serving.