Make the salt water. Warm up 90 ml of water just enough that the salt will dissolve in it easily. Dissolve the salt in it, and leave the water to cool down to room temperature. (10 g of salt is 2 teaspoons)
Combine the flours in a bowl. Add the salt water to this little by little while you mix it into the flours. (Don't worry if it looks like there's not enough water in the dough at this stage.)
Form the dough into a ball, cover completely with plastic wrap, and rest at room temperature for 30 minutes. (The flour will become permeated with the moisture in the dough as it rests.)
Put the dough in a doubled up thick plastic bag (or two plastic bags). Step on the dough evenly, starting from the middle and working towards the edges.
When the dough is flat, take it out of the bag, fold up up into a small bundle again, return to the bag and step on it until it's flattened. Repeat about 5 more times.
If you have the time, keep folding and stepping on it again and again. Step on it with your kids, step on it with your hubby, step on it with your lover.
When you've stepped on it as much was you want, roll the dough up, wrap it in plastic and rest it for 30 minutes to an hour. (You can skip this resting step, but it's better if you don't.)
After the dough has rested, flour a cutting board and roll the dough out very very thinly with a rolling pin.
Dust with more flour and fold the dough, and slice it into noodles from the edge.
Separate the cut noodles as you dust it with more flour.
Boil the udon noodles in plenty of water. (If you're serving them cold, cook for a slightly longer time.)
Rinse the cooked noodles well in cold water until the surface stickiness is gone. Serve with toppings of your choice.
Story Behind this Recipe
There are many different ingredient ratios for making udon noodles. I tried several versions, and of them all I thought this one is the best, so I decided to upload it.
You don't have to buy medium-strength or all-purpose flour just to make udon noodles. (Translator's note: all-purpose flour is not usually stocked in Japanese home kitchens, but bread and cake flour often are.) It's often written that you need to adjust the amount of water used from 40 to 45% depending on the season, but I find that dough with just 40% water is too stiff and hard to knead! Dough made with 45% water is still very delicious. In the wintertime, try to rest the dough in as warm a place as possible.