Start by lining your square baking pan with parchment paper and set aside.
In a small mixing bowl, stir together the butter, brown sugar and cinnamon until you have a smooth spreadable paste.
Remove the dough from the refrigerator and scrape out onto a well floured work surface.
Press the dough into a rectangle and then roll out to be a large 12" wide by 15" long rectangle.
Spread the cinnamon sugar butter over the surface of the dough, leaving a small gap around the edges.
Then reserve about 2 tablespoons of the blackberry compote for the frosting and spread the rest over the cinnamon butter.
Roll the dough from the bottom to the top into a log.
Using flavorless dental floss or thin kitchen twine, trim the ends of the log. Remove about an inch or so off each end to cut off uneven and unfilled dough.
Using the floss or twine, divide the remaining log into 9 rolls and place these in your lined baking pan.
Cover the pan with a clean kitchen towel and allow to proof at room temperature for about 2 hours until doubled in size.
Towards the end of this second proof, preheat your oven to 350ºF and make a quick egg wash using the reserved egg yolk and a splash of milk.
Brush the rolls with the egg wash and bake in the centre of the preheated oven until risen and golden brown (about 25-30 minutes).
Remove from the oven and allow to cool for about 10 minutes while you make the frosting.