Put 250g blueberries in a small pot. Add 125g sugar, 3 tbsps cornstarch and 50g water.
Mix to combine everything. Bring the mixture to a boil.
Let simmer for about 10 minutes. Turn off the heat and let the mixture sit in the pan for about 15 minutes to a cool a bit.
Add the mixture to a bowl and cover with aluminum foil. Let cool in the refrigerator until needed.
Mix the dough
Mix the dough when your starter is ready. Add 450g bread flour, 150g sourdough starter, 4 eggs, 30g sugar, 15g salt and 60g milk to a bowl.
Put your index and long finger in the middle of the dough and start moving it in circles; for each revolution, make the circle a little bigger, so you get more and more flour into the fluids.
Once that gets difficult, grab the back of the dough with your hand, stretch it up and in over the dough, turn the bowl a bit and repeat until all the flour is completely hydrated.
Leave the dough to rest for 30 minutes under a damp dishtowel.
Bulk fermentation
Perform four sets of stretch and folds spaced out by 30 minutes.
Now it's time to add the butter. Put a small handful of cubes on the dough, push it and massage it into the dough. Keep going until you cannot feel any lumps anymore. Then add another handful. Repeat until you have added all the butter.
Then let the dough ferment until it has expanded by 30-50% in volume.
Put the dough in the refrigerator for a minimum of 2 hours and a maximum of 48 hours.
Shape and final proof
Flour your work surface liberally. Dump the dough out and tease it into a rectangle. Roll it out so that it's about the width of your pan minus 1.5cm/½". Roll it thinly, so the dough has to roll several times to be a cylinder.
Apply the blueberry jam to the dough. You can go to the edge, at the end where you will start rolling, but at the other end, you should leave about 3cm/1" room without any jam. On the sides, you should leave about 1.5cm/½" room without jam.
Then roll up the dough pretty tightly. When you have a dough roll, fold the ends underneath the bottom to seal the jam inside.
Grease your tin with butter or baking spray and place the dough in the tin. Then cover the tin with a damp dishtowel and let it proof until it has doubled in size.
Bake the bread
Heat the oven to 200ºC/390ºF/Gas mark 6.
Beat one egg with one tablespoon of milk. Brush the bread lightly with the egg wash.
Bake for about 60 minutes or until the inner temperature reaches 98°C/208ºF. If the bread starts to get too brown during baking, cover it with aluminum foil.
Once the bread is out of the oven, let it cool for 10 minutes and then take it out of the tin and let it cool completely.