Preheat the oven to 425°F, with a rack in the middle position. Put a rimmed baking sheet in the oven to preheat. Peel and grate the potatoes, and transfer to a large bowl. Cover with boiling water by two inches. Let stand for 10 minutes. Drain the potatoes in a colander and rinse thoroughly with cold water. Squeeze the potatoes to remove excess moisture, and transfer back to the large bowl. Add the flour, cornstarch, salt, pepper, and egg yolk and fold into the potatoes.Spread the panko on another rimmed baking sheet. With wet hands, pinch off tablespoons of the potato mixture, form into balls, and coat with the panko crumbs.Carefully remove the hot baking sheet from the oven and coat with 2 tablespoons of the olive oil. Quickly transfer the poppers to the baking sheet and drizzle with the remaining 2 tablespoons olive oil. Bake for about 30 minutes, flipping once, until golden brown and crispy. Serve with ketchup.