Melt the butter in the microwave (or in a bain-marie, if you prefer) and let it cool slightly.
In the meantime, whisk the eggs with the sugar, then add the butter, cinnamon and pumpkin to the eggs and mix.
Sift the flour together with the yeast; first add half the flour to the egg mixture, then the milk, and finally the remaining flour.
Transfer half the mixture into the buttered and floured mould, then incorporate the sifted cocoa into the remaining mixture and add it to the mould.
Mix with a spoon to create the marbled effect, then bake in a preheated static oven at 170°C and cook for about 40 minutes (be sure to always do the toothpick test before taking out of the oven).
Once the cake is out of the oven, let it cool, then prepare the icing: place the chocolate pieces in a saucepan together with the milk and cook in a bain-marie until it melts completely.
Finally, unmold the cake, place it on a serving plate and decorate the surface with the ganache and coarsely chopped hazelnuts, then leave to dry for a while.
The marbled pumpkin donut is ready, all you have to do is cut it into slices and serve it.