In a small saucepan, combine the dashi broth, soy sauce, mirin, sake, and sugar. Bring the mixture to a boil, then reduce the heat and let it simmer for a few minutes until the flavors are well combined.
In a shallow dish, toss the cubed tofu in the cornstarch until it's well coated.
Heat the vegetable oil in a deep frying pan over medium-high heat. Once the oil is hot, carefully add the tofu cubes to the pan and fry them until they're golden brown and crispy on all sides.
Use a slotted spoon to remove the tofu cubes from the pan and drain them on a paper towel.
Divide the fried tofu cubes among serving bowls. Pour the hot dashi broth over the tofu cubes, and garnish with sliced green onions and grated daikon radish, if desired.
Enjoy your homemade agedashi tofu with a warm bowl of rice or a side of steamed vegetables. It's a comforting and flavorful dish that's perfect for a cozy night in.