Combine the milk, water, egg, oil, butter, powdered sugar, yeast, salt, and flour. Knead until you get a soft dough. If it is sticky, add a little more flour until it becomes smooth.

Divide the dough into two equal parts. Add the cocoa powder to one half and knead until fully combined. Let both doughs rise for about 40 minutes.

Roll out the plain dough and place the cocoa dough in the center. Fold the plain dough around it and seal it well. Roll it out again into a large circle.

Cut the dough into triangles. Place Nutella on the wide end of each triangle and roll up into croissant shapes.

Place the croissants on a baking tray and let them proof for about 30 minutes.

Brush with egg yolk and bake at 180°C for about 30 minutes, or until golden brown.

Let the croissants cool slightly, then dust with powdered sugar before serving.