Bring a pot of water to a boil, generously season with salt and cook the potatoes until fork tender, about 10-12 minutes or longer as needed. Drain the cooked potatoes and transfer to a paper towel lined baking sheet to air dry for 10-15 minutes.
Transfer cooled potatoes into a large mixing bowl. Use a potato masher to mash until smooth. Mix in the cornstarch and seasonings (garlic powder, onion powder, paprika, salt and black pepper).
Spoon the mashed potatoes onto a large parchment lined baking sheet. Top with another sheet of parchment paper. Use a rolling pin to form the mashed potatoes into a rectangular shape measuring roughly 8x9.5 inches.
Transfer baking sheet to the fridge and refrigerate the shaped mashed potatoes for at least (2) hours or overnight.
Heat oil to 350F. As the oil heats, cut chilled mashed potatoes into long fries starting on the short end of the rectangle. In batches, fry the strips of potatoes until crispy and golden brown, about 4-5 minutes or longer as needed. Transfer to a wire rack placed over a paper towel lined baking sheet to catch any excess oil.
Drizzle long fries with Japanese mayo and sriracha and serve immediately while hot and crispy. Enjoy!