Sift the flour into a mixing bowl. Add the caster sugar and the yeast and mix it together.
Rub the butter into the flour until it resembles breadcrumbs.
Next, warm the milk either in the microwave or a small saucepan. You do not want it to boil.
Add 2/3 of warmed milk to the flour mixture and then add in the egg and the vanilla extract. Stir to thoroughly combine. Use more of the milk if you need to, to get the mixture to come together with no floury bits left in the bowl. Different flours absorb liquids differently so you may not need it all but if you do - use it. Just go slowly.
Gather the mxture together and tip it out onto the work surface and begin to knead the dough. The dough may feel a little sticky at first but it will begin to become smooth and elastic. This may take between 5 - 10 minutes.
As you knead the dough it will eventually become springy, smooth and no longer sticky.
At this point, put the dough into a clean bowl which is lightly oiled. Cover it with clingfilm (plastic wrap). You want the bowl have enough space for the dough to double in size without the clingfilm touching the dough.
Leave the dough to prove for 1 -2 hours or until it doubles in size.
Grease your baking pan so that's ready for the buns later.
In the meantime make the cinnamon filling. Mix the muscovado sugar and the cinnamon spice together. Melt the butter for the filling.
Once the dough has risen, tip it out on the work surface and roll it out into a rectangle using your rolling pin. This rectangle needs to be about 50cm by 30cm.
Use a pastry brush and spread the melted butter over the rolled out dough and then sprinkle it with the sugar cinnamon mixture. Try to do this evenly.
Time to roll up the dough. Roll the rectangle long side to long side. Just like a swiss roll. Take your time with this and roll it as tightly as you can without stretching the dough.
Now slice the rolled dough into 12 equally sized pieces. (The ones at the ends are likely to be a little smaller but that's ok)
Grease your baking pan so that you can get the buns out easily.
Place the rolls gently into your baking pan. (Size 24cm by 30cm) They need a little space around them so that they have space to expand. At this point they do not need to touch. Cover lightly with clingfilm and allow them to prove again.
Leave the rolls for about another hour to rise. At the end of the hour the rolls should just be touching. During this hour, pre-heat the oven to 360F/180C/160C Fan
Bake the buns for 20 minutes until they are golden brown.
Once baked take them out of the oven and allow them to cool in the pan for around 10 minutes before adding the glaze.
Allow the buns to cool for another 20 minutes before removing them from the pan. Now you can try one. Enjoy!