Prepare the udon dough: place the udon flour and salt in a bowl and blend well. Add the water to the bowl and knead until the dough is firm and smooth. The dough should feel like an earlobe when it has been kneaded enough.
Transfer the dough onto a clean work surface: pound the dough flat using a rolling pin. Once the dough is flat, fold the dough back and pound flat again. Repeat this procedure 5 times to develop the gluten.
Set aside the dough to allow it: cover the dough in cling film and set aside at room temperature for 2 hours.
Remove the plastic wrap and place the dough on the work surface: sprinkle some potato starch on the surface to keep the dough from sticking. Roll the dough out to 1cm thickness using a rolling pin. Fold the dough into thirds and cut dough into 1cm thick and 1cm wide strips.
Cook the udon noodles: cook the udon in boiling water for 10 minutes, until al dente. Drain well in cool water and set aside.
Prepare the noodle soup: combine the 400ml dashi soup base, 1 tbsp mirin, 1/2 tsp sea salt and 1 tbsp soy sauce in a saucepan over medium heat and bring to the boil. Beat 2 eggs in a measuring jug. Stir the soup in a circular motion and add the eggs slowly into the boiling soup using a swirling motion to make thin ribbons. Turn the heat off immediately once the egg ribbons float to the surface.
Garnish and serve: place the udon in a bowl and add the egg drop soup. Garnish with spring onions and serve with shichimi chilli powder if desired.