First, dissolve the yeast in the freshly warmed milk, then pour it onto the flour together with the sugar and vanilla and start working.

Add the salt and half the butter cut into pieces; once completely incorporated, add the remaining butter, again in small pieces.
Cover the bowl with a clean cloth and let rise for about 2 hours or until it has at least doubled in volume.

Once completely leavened, take the dough again: for convenience I divided it into 2 parts, so that the work was easier, I suggest you do the same, but it is not mandatory.
Roll out the first piece of dough into a fairly thin sheet (3-4 mm) and divide it in half vertically, then in half again horizontally and then again obliquely from both sides, so as to obtain 8 wedges.
Place 1 teaspoon of Nutella on the wide side and roll the dough over the filling until you reach the tip.
Proceed in the same way to form all 8 croissants and then similarly with the second piece of dough, so as to obtain a total of 16 croissants in the end.

Once all the croissants have been shaped, place them in a mold lined with baking paper: the mold must be large enough to contain them all, leaving a little space between one and the other, but not too large, so that after leavening they uniscano (mine was perhaps slightly too small).
Leave to rise for another hour in a warm place and, when they are nice and puffy, cook for about 25 minutes in a preheated static oven at 180°C.

Your croissant cake is ready, all you have to do is let it cool, decorate with icing sugar and serve.