Combine dry ingredients - Add the bread flour, yeast, brown sugar, cinnamon, allspice, nutmeg, milk powder, salt and orange zest to the bowl of a stand mixer fitted with a dough hook. Use the dough hook to mix everything until well combined.
Mix wet ingredients - In a large jug whisk together the eggs, milk and vanilla extract.
First knead (12-15 min) - Add the milk mixture and cooled Tangzhong to the dry ingredients. Use the dough hook to mix the ingredients. Turn the mixer on 30% (that’s speed two on my Kenwood) and knead for 12-15 minutes. The dough will look smooth and come away from the sides of the bowl.
To test if dough is kneaded enough - Test by stretching a small piece between your fingers. It should form a thin, see-through sheet without tearing. Kneading time can vary depending on your mixer so focus on the dough’s feel rather than just time.
Rest dough (10 minutes) - Turn the mixer off and cover the bowl with plastic wrap. Rest the dough for 10 minutes. This will help relax the gluten in the dough and gives you an even smoother more elastic dough that is less sticky to work with later.
Add the butter and finish kneading (9-10 minutes) - Begin kneading the dough on 30% / speed 2 again for about 3 minutes then add the softened butter a little at a time, letting each piece mostly mix in before adding the next. This will take about 2-3 minutes all up. Once all the butter is added, knead on medium low speed for a final 2-3 minutes until the dough is smooth, elastic and comes away cleanly from the bowl. A slight stickiness at this stage is normal. If the dough begins looking overly shiny as the first bit of butter is added, it’s too warm. Cool the dough down in the fridge for 10 minutes before adding the remaining butter.
Add sultanas (90 seconds) - Add the sultanas and mix on low speed for about 90 seconds until evenly distributed. You can also knead these in by hand.
Room temp rise (Proof #1) - Lift the dough out of the bowl and shape into a smooth ball. Then place back in the bowl (you can transfer it to a large mixing bowl if the stand mixer bowl doesn’t fit in your fridge). Cover the plastic wrap and let it rest at room temp for 1 hour. It will more than double in size.
Cold rise (Proof #2) - Place in the fridge for at least 3-4 hours (overnight gives you better flavor!). The dough will continue to expand as it chills and develop lots of great flavor! The goal here is to chill the dough so that its easier to shape.
Prepare baking tin - Rub a cold stick of butter on the bottom and sides of a 9x13-inch / 23x33cm baking tin and line with baking paper.
Weigh out buns - Dust your workbench with flour and pour the risen dough onto the bench. Knead a little to form a large ball. Divide the dough into 12 even sized portions by weighing them using a kitchen scale. Each portion should weigh about 90g. This will ensure even sized buns.
To form dough balls - Place a cupped hand over one dough piece and swirl around on the bench top until it forms a ball. The slight stickiness of the dough will grip onto the work bench and help shape it under your hand forming a tight ball. Place the dough piece in your baking tin with the smooth side up. Repeat this until you’ve shaped all your buns.
Final rise (Proof #3) - Cover loosely with plastic wrap and let them rise for 2-3 hours. It could take more if it’s cold! They will have more than doubled in size before they’re ready to do in the oven and will be slightly jiggly when the tin is shuffled a little. I can tell they’re done when they’ve expanded so much they’re touching each other in the sized tin I use.
Preheat oven - 15 minutes before you’re ready to bake, preheat your oven to 195C / 380F (fan off), 175C / 350F (fan on).
Flour paste - Add the flour, water and melted butter to a small bowl to form a paste. Transfer to a piping bag and snip the end off. Pipe X’s onto the buns. Get them nice and thick so they don’t bake up chewy in the oven.
Bake - Bake in the middle rack of your oven for 30-32 minutes or until golden brown on top.
While the buns are baking make the syrup - You’ll need 100 of liquid all up for the syrup. Add the freshly squeezed orange juice into the pot first, then add water to get it to 100g. Add the sugar and cinnamon stick in as well. Bring to a boil and then turn the heat down to low and simmer for 5 minutes. Allow to cool.
Brush syrup on buns - When the buns come out of the oven, use a pastry brush to brush with the syrup. Allow to cool until warm before serving.
To reheat - These can easily be reheated in the oven for about 5 minutes or until warm and soft again, but they stay soft for days if stored in an airtight container.