Add blueberries, sugar, and lemon juice to a small saucepan and heat, stirring occasionally. Let simmer for about 10–15 minutes until the blueberries are soft and the jam has a thick consistency. Strain and let cool.

Sift flour into a bowl and make a well in the center. Place the yeast and 1 tsp sugar in the well, add 3 tbsp lukewarm milk, mix lightly, and let it rest for about 10 minutes until bubbles form.

Add the remaining sugar, salt, milk, softened butter, and egg to the yeast mixture, and knead until a smooth, elastic dough forms. Cover and let it rise in a warm place for 1 hour, or until it has doubled in size.

Roll out the dough on a floured surface into a rectangle, about 25 x 20 cm, and spread the cooled blueberry jam evenly over it.

Fold the top of the dough to the center, then fold the bottom over it to form a long rectangle.

Cut the folded dough into 9 even strips, each about 4 cm wide. Gently take each strip, stretch it slightly, and roll it up.

Place the rolled strips in a greased or parchment-lined baking pan, press down lightly with your hand, and let rise again, covered, for 30 minutes.

Preheat the oven to 180°C (350°F). Brush the brioche rolls with egg yolk, sprinkle with almond slivers or chopped hazelnuts, and bake for about 25–30 minutes, until golden brown.

After baking, cover the brioche rolls with a clean cloth to maintain their soft, fluffy texture. Once cool, dust with powdered sugar if desired.