Make the batter by mixing together the flour, sugar and salt in a large bowl. Make a well in the centre and add the eggs and a splash of milk. Whisk together and gradually add the milk to make a smooth batter.
Brush a frying pan with oil and ladle in a thin layer of batter and fry the crepes one by one until the batter is all used up (should make about 30.) Stack on a plate and set aside.
Pour the hot cream over the chocolate and allow to melt together, then set aside to cool to a spreadable consistency.
Whisk the double cream to soft peaks, add a couple of spoonfuls to the chocolate spread to loosen, then add back into the bowl and continue to whisk until stiff.
Assemble by layering each crepe with a thin layer of chocolate cream. Trim the edges so they are neat, then top with the ganache. Top with toasted hazelnuts.