Drain and wrap the tofu in paper towel. This will remove excess water. Cut into quarters and set aside.
In a small-medium size pot, add 4 cups of water and bring to boil. When the water is boiling, turn off the heat and move the pot to a cool surface.
Mix the potato starch with 1 tbsp water, add to the pot and stir well.
Gently put the eggs in the pot, cover and set the timer for 8 minutes.
In four separate bowls, put arugula at the bottom and top with silken tofu.
Make the tsuyu by combining soy, mirin, bonito flakes and sake. Microwave for a minute and set aside.
When the eggs are done cooking, carefully transfer them in a bowl of cold water and let cool for a minute.
Very gently break each eggs in a small bowl, then lay on top of each piece of tofu. This prevents the eggs from sliding to the bottom when breaking them right on top of the tofu.
Add scallions and ginger and finish by pouring a little tsuyu on top. Serve immediately.