Add the butter a few cubes at a time, mixing until incorporated. Knead for 10 minutes, until the dough is smooth and elastic. Cover and let rise until puffy, about 1.5 hours. For easier handling, refrigerate the dough overnight after its initial rise, if desired.

Divide the dough into four portions, about 14 to 16 ounces/400 to 450 grams each. Working with one portion at a time, roll the dough into a thin, elongated rectangle on a lightly floured surface.

Spread generously with chocolate hazelnut spread. Fold the dough over itself, keeping the seam along the side, then roll it out again and spread with another generous layer of chocolate spread.

Roll the dough into a log and place it seam-side down on parchment paper. Using a long, sharp knife, cut the log lengthwise in half. With the cut sides facing up, twist the two strands together to form a braid.

Transfer the braid to a parchment-lined metal loaf pan. Cover loosely with plastic wrap and let rise until doubled in size. Repeat with the remaining dough.

Heat the oven to 350°F/175°C. Bake the loaves on the lower rack for about 25 minutes, until golden and baked through. If baking multiple loaves at once, rotate the pans as needed for even browning.

Let cool in the pans before slicing. Store at room temperature in a cool environment or refrigerate for longer storage. Warm briefly before serving, if desired.