Place all the ingredients except the butter in your stand mixer using a dough hook (or in a large bowl if making by hand).
Mix gently to combine.
Add the butter gradually during the following kneading stage.
Knead for approx. 8 minutes until smooth and elastic, and the dough comes clean from the side of the stand mixer bowl.
Remove the bowl from the mixer, cover bowl with clingfilm and leave to rise for 2 hours or until doubled in size.
Push back the dough to deflate it, then roll it out to a large rectangle.
Sprinkle the blueberry powder all over the dough to cover it.
Fold in the sides of the dough to meet in the centre, then roll the dough out again into a longer rectangle. Fold the top and bottom down again to meet in the centre, then fold the bottom half onto the top half. This is similar to the folds you carry out when making puff pastry. I did this twice.
Roll the dough out again into a rectangle shape. Roll it up along the wide side like when making a Swiss roll. Cut into six slices.
Using butter or cake release spray, prepare a cake tin. Place the slices in the tin and cover with clingfilm. Leave to double in size again, for around 45 minutes.
Preheat your oven to 180 degrees C (fan). Bake for 20-25 minutes until golden brown and cooked through.