Pour the soy milk into a mug and mix in however much nigari on the package instructions. Then put it in the microwave.
This should take just under 2 minutes in a 600 W microwave. Keep peering into the microwave and when the mixture starts to bubble and boil, take it out. It should have set a little bit already.
Quickly cover the mug with cling film, wrap the mug in a towel or an oven mitten to retain the heat and leave it. You could also cover it with a tea cozy or tea pot.
Wait 10-15 minutes and your homemade tofu is ready! Take it out of the mug and serve. Or you could place it on a fancy decorative sieve and present it another way as "zaru" (sieve) tofu!
You can eat this tofu still warm or chill it and serve it cold. Both ways are delicious. I recommend eating this with herbs and a pinch of salt for flavor.
Story Behind this Recipe
I developed this recipe in response to the well-loved "Custard Pudding in a Mug" (Recipe ID: 1351368). I was originally thinking of making a dessert but decided to create a tofu recipe instead. The way of making it is almost the same but far simpler! Homemade tofu really showcases the delicious flavour of the soy beans, so give it a go!
I usually put the mixture in the microwave for about 1 minute 45 seconds before it boils. The boiling time also depends on the temperature of the soy milk, so once you can see it boiling, stop the microwave. It will continue cooking with the residual heat anyway. The tofu will set in residual heat so there's no need to add vinegar to get a smooth and soft tofu. If the tofu turns out too watery and crumbles, add it to a sieve and drain the water away to prepare "zaru" tofu.