Add both flours and your salt to a large bowl and mix together.
Rub in your softened butter, you can do this by hand or in a food processor.
Stir in your sugar.
Heat your milk until just warm, add your yeast to it and stir in,. Don't worry if it remains a bit lumpy.
Beat together one whole egg and one egg yolk.
Make a well in the centre of the flour and pour in the milk mixture and the beaten eggs.
Mix together in a stand mixer with a dough hook fitted on a low-medium setting for 2 minutes (lowest setting if using a KitchenAid). Or knead by hand for 8 minutes. The dough is ready once it sticks to the dough hook in a clump, it is slightly sticky and also stretchy and smooth.
Remove the dough from the bowl, oil the bowl with some flavourless oil (or grease with butter), return the dough to the bowl and then cover the bowl with an oiled piece of clingfilm (plastic wrap).
Leave in a warm place to prove for 1½-2 hours, until doubled in size.
Whilst your dough proves, make your blueberry jam, so that it has time to cool.
Place a small plate into your fridge or freezer to chill.
Place your frozen blueberries, granulated sugar and juice from half a lemon into your small saucepan and heat on a medium setting, whilst stirring, until the sugar has all dissolved.
Turn up the heat and bring to a rolling boil.
Turn the heat down a little and fast simmer on a medium-high temperature, without stirring. After 15 minutes place a teaspoonful of the jam onto your chilled plate. If ready it should spread out just a little and then set. If that does not happen and the jam juice is still very thin and runny, then continue simmering for another 5 minutes and then test again.
Once ready remove from the heat and leave to fully cool (spread out on a plate if you want to speed up the cooling).
After your dough has risen remove it from the bowl and gently knead it by hand on a lightly floured work surface just 2-3 times.
Grease and line the base of your roasting tin.
Roll the dough out to a large rectangle measuring approximately 52cm x 32cm. You may need to keep shaping the sides with your hands to ensure you end up with a rectangle.
Mix together your brown sugar, soft butter and cinnamon to form a paste.
Spread the cinnamon filling out onto your rolled out dough, making sure you get right to the edges.
Then spread over about two thirds of your cooled blueberry jam.
From the shortest side, gently and tightly roll the dough up.
Using a sharp serrated bread knife, cut the roll in half, then cut each half in half and then each of the quarters into three equal-sized rolls, so that you have 12 rolls. You will need to wipe your knife and coat it in flour between each cut, to keep it clean and thus the buns clean from jammy smears.
Add the rolls to your greased and lined baking tin in 3 rolls of 4 (4 on the longest side), making sure they are equally spaced so they have room to expand whilst they prove.
Again, cover with oiled cling film and leave to prove for 30-45 minutes, until they are all touching.
Heat your oven to 180°C/160°C Fan/350°F/Gas Mark 4
Bake in the middle of your oven for 20-22 minutes, until just golden all over.
Leave to cool in the tin.