Mix the lukewarm milk, yeast and 1 tsp of the sugar. Leave for 5–10 minutes until foamy.
In the bowl of a stand mixer, mix the flour, remaining sugar, vanilla sugar and salt.
Add the eggs and yeast mixture and knead with the dough hook for about 5 minutes.
Add the soft butter 1 tbsp at a time, kneading until each piece is mixed in before adding the next.
Knead for another 8–10 minutes until the dough is very smooth, soft and elastic. It will be sticky at first. Don’t add extra flour.
Cover and leave to rise in a warm place for 60–90 minutes, until doubled.
Divide the dough into 12–14 equal pieces. Flatten each one, place a generous tablespoon of Nutella in the centre and pinch firmly closed.
Pour the 40 g melted butter into the baking dish and spread it around. Place the buns seam-side down, close together.
Cover and leave to rise for 35–45 minutes, until very puffy and touching. Preheat the oven to 175–180°C (350°F), conventional.
Brush with the remaining melted butter and bake for 25–30 minutes until golden. Cover loosely with foil if they brown too quickly.
Cool in the dish for 10 minutes, dust with powdered sugar and serve warm.