Into the bowl of a stand mixer, place the flour, yeast, sugar, salt, milk powder (if using), egg, yolk, vanilla, orange zest, spices & the cooled down tangzhong.
Next, warm the milk to 38°c/100°f (either in the microwave or on the stove) then add into the flour.
Using the dough hook, mix the dough on a medium speed for 4-5 until smooth then mix the softened butter in, a small piece at a time. Make sure that each piece of butter is fully incorporated before adding in more.
Continue mixing on a medium speed until the dough passes the window pane test. This will take around 5 minutes.To perform the window pane test, stretch a small amount of dough between your fingers. If the dough stretches thin enough so that you can see your fingers through it without tearing, it's ready. Next, drain the soaked fruit through a sieve, discarding the liquid then add the fruit into the dough along with the mixed peel. Mix on a medium-low speed until fully incorporated (3-4 minutes).
Transfer the dough to an oiled mixing bowl then cover & leave to prove at room temperature, until doubled in size. This will take 1-1½ hours.
Once the dough has risen, knock it back, tip out onto a clean work surface then divide into 12 equal pieces, using a bench knife. Each piece should weigh around 100 grams.
Shape each piece of dough into a ball then place onto a lined baking tray, in 4 rows of 3, leaving a couple of centimetres between each bun. Cover the tray lightly with clingfilm (or an upside down roasting tin) & leave to prove for another 1-1½ hours, until almost doubled in size.The buns should have risen by about 75% & should just be touching. Whilst your buns are rising, preheat an oven to 180°c/160°c fan (356°f/320°f).