Dissolve yeast and granulated sugar in 1⁄4 cup warm water in a large bowl; set aside to let sit until frothy, about 10 minutes. Add half of the brown sugar, milk, butter, syrup, eggs, and yolks to yeast mixture; stir well with a fork. Add flour and salt; mix with your hands into a rough ball. Transfer dough to a lightly floured surface; knead until smooth, 3–4 minutes. Grease a large bowl with 1 tbsp. of the oil; nestle dough inside. Cover bowl with plastic wrap; set aside to let rise in a warm spot until dough doubles in size, about 1 hour.
Preheat oven to 180°C. Transfer dough to a lightly floured surface and knead gently for 30 seconds. Roll into a 25*40cm rectangle. Sprinkle remaining brown sugar and blueberries over top and roll up very tightly lengthwise, tucking in any escaped filling. Grease two 6-muffin tins with remaining oil (don't use a 12-muffin tin, or the brioche in the middle won't have room to expand). Cut dough crosswise into 12 equal rolls. Place rolls, flat side up, in tins, gently squeezing them to fit. Set aside, covered with plastic wrap, to let rise for 15 minutes.
Uncover and bake brioche until cooked through and light golden, about 35 minutes. Unmold onto a rack to let cool; dust with confectioners' sugar.