Put the rice vinegar, sugar, salt and powdered Dashi into a small saucepan.
Heat gently and stir well to make sure that the sugar and salt dissolve. Don't let the mixture boil, only heat it long enough to completely dissolve all the ingredients.
Turn off the heat, and leave it to cool while you prepare the rice.
Rice
Wash the rice very well under cold water. Make sure the water comes out transparent at the end of the wash.
Put the rice in a large bowl and soak it for 30 minutes.
After the 30 minutes have passed, drain the rice and put it in a pot. Add the 1 ¾ cups of water to it. Cover it with a tight lid and put it on a high flame until it starts boiling, then simmer it on low for 10 to 12 minutes or until all the water has been absorbed.
When ready, let it rest for 10 minutes BEFORE removing the lid.
Scoop the rice out onto a big plate, season it with the sushi seasoning and use a spatula in a cutting motion to fold the seasoning into the rice. While you do this, use your other hand to fan the rice. Keep fanning and folding until the rice has reached room temperature.
If you are not ready to use the sushi rice immediately, cover the plate with a damp cloth. This will prevent it from drying out.
Now you can use this rice to make sushi.
Recipe Notes
You can substitute all the ingredients marked with a * with ready made Sushi Seasoning (the brand I used called for 3 tbsp per 1 cup of cooked rice, so 15 tbsp in total for the above recipe).