Preheat the oven to 180C/356F and line a 24 x 24cm (9.5 inch x 9.5 inch) square brownie/cake tin with baking paper, leaving some overhanging to act as handles later.
Combine the boiling water and yoghurt in a medium mixing bowl. Add the psyllium husk and whisk vigorously until there are no lumps remaining. Set aside for 15 minutes to form a gel.
In a large mixing bowl, whisk together the remaining ingredients for the buns, except for any mix ins you are using.
Once the gel is ready, pour it into the dry ingredient bowl along with the oil. Use a spoon to combine most of the way, then get your hands in and squelch the dough through until all the flour has been picked off up the bottom of the bowl.
Continue until the dough is a uniform light brown with no light patches from the yoghurt and no lumps of flour. This will take about 5 minutes but is necessary for an edible bun, so keep going until you get a uniform consistency.
I find this dough works best when it can hold it's own shape but is too sticky to roll or hold in a dry hand. If yours is drier than this, adjust as necessary. If yours is wetter, I'd suggest leaving it. Your buns might spread into one giant bun, but they will be nice and moist.
Add the mix ins, if you are using them. Use your hands to distribute them through the dough.
Thoroughly wet your hands with water, then divide the dough into 9 balls. I do this on the bench so I can eyeball that they are roughly the same size.
Thoroughly wet your hands again, then roll each piece of dough into a ball with a smooth top. The smoother the top is before baking, the nicer the bun will look after baking.
If you only have a baking dish that is larger than the one specified, use smaller dishes to tuck the buns in tightly. They will spread if left to their own devices.