After the first proof, scrape your dough out of the bowl and onto a well floured work surface. Press the dough into a rough rectangle and then roll out with a floured rolling pin to be about 12" wide by 15" long.
Dollop softened butter over the surface of the dough and spread in an even layer.
Mix together the ground cinnamon and remaining 2 tablespoons of sugar. Sprinkle this over the butter and then add the blueberry filling, spooning over the top. (see note)
Begin rolling the dough from the bottom to the top, forming a log of dough. Then using flavorless floss cut about 1½" off each end, trimming the uneven dough.
Divide the dough into 9 and use the floss to cut. (see note)
Place the rolls in a parchment lined 8" square baking pan and cover with a clean kitchen towel to proof for about 2 hours until doubled in size.
Near the end of your second proof, pre-heat the oven to 350°F (175C/155C Fan) and prepare an egg wash by lightly beating together the reserved egg yolk with a splash of milk.
Brush the rolls with the egg wash and place in the centre of the preheated oven to bake for about 25 minutes until risen and golden.
Remove from the oven and allow to cool for about 10 minutes while you make the glaze.