Prepare the yeast mixture: Combine the lukewarm milk, sugar, and instant yeast in a large mixing bowl. Stir gently and let the mixture rest for 5–10 minutes. It should begin to look slightly foamy.
Add the wet ingredients: Add the eggs, melted butter, and vanilla sugar or vanilla extract. Whisk until everything is evenly combined.
Add the flour: Add the salt and gradually mix in the flour. Continue mixing until a soft, slightly sticky dough begins to form. Avoid adding all the extra flour unless it is necessary.
Knead the dough: Transfer the dough to a lightly floured surface and knead for 8-10 minutes, or use a stand mixer fitted with a dough hook. The finished dough should be smooth, elastic, and soft.
Complete the first rise: Shape the dough into a ball and place it in a lightly oiled bowl. Cover with plastic wrap or a clean kitchen towel. Let it rise in a warm, draft-free place for approximately 1 hour, or until doubled in size.
Roll out the dough: Gently press the risen dough to release excess air. Transfer it to a lightly floured work surface and roll it into a long, evenly shaped rectangle. Roll it slightly thinner for more defined chocolate layers or leave it a little thicker for softer, bread-like buns.
Add the chocolate filling: Spread the chocolate hazelnut spread evenly over the dough, leaving a small border around the edges. The spread should be at room temperature so it can be applied without tearing the dough.
Fold and cut the dough: Fold the dough in half to enclose the chocolate filling. Press it gently so the layers hold together, then use a sharp knife or pizza cutter to cut it into 9 evenly sized strips.
Shape the buns: Gently twist each strip along its length. Coil the twisted strip into a loose spiral and tuck the end underneath the bun to prevent it from opening.
Complete the second rise: Arrange the shaped buns in a baking tray or pan lined with parchment paper, leaving some space between them. Cover loosely and let them rise for 20-30 minutes, or until soft and puffy.
Bake: Preheat the oven to 400°F (200°C). Bake the Chocolate Swirl Buns for 20-25 minutes, or until lightly golden on top.
Finish the buns: Brush the warm buns with melted butter immediately after removing them from the oven. Let them cool slightly, then dust with powdered sugar if desired.