Combine the bread flour, all-purpose flour, sugar, instant yeast, and fine salt. Add the lukewarm milk and mix until a rough dough forms.

Knead the dough for 8–10 minutes, then work in the softened unsalted butter. Continue kneading until smooth, elastic, and only slightly tacky.

Cover the bowl and let the dough rise in a warm place for 45–60 minutes, or until it is nearly doubled in size.

Divide the dough into 8 equal pieces, shape each piece into a ball, cover, and let rest for 10 minutes.

Roll each ball into a long triangle approximately 20–25 cm (8–10 inches) long. Place one chilled salted-butter baton at the wide end.

Roll the dough tightly from the wide end toward the point, keeping the butter enclosed. Seal the tip, place each roll seam-side down on a lined baking sheet, and cover.

Let the shaped rolls proof for 25–40 minutes, until visibly puffy but not fully doubled. Meanwhile, heat the oven to 200°C (400°F).

Brush the rolls lightly with the egg wash and sprinkle with 1–2 teaspoons of flaky sea salt. Bake for 12–16 minutes, until golden brown, then serve warm.