Line a baking tray with parchment paper.
Heat 1 cup milk in a non-stick pan. Add sugar and cook till it melts.
Take fresh yeast in a bowl. Add ¼ of the warm milk and mix well. Set aside for 1-2 minutes to activate.
Add butter to remaining milk and whisk well.
Sieve flour in a bowl. Add activated yeast, 1 egg yolk and 1½ teaspoons vanilla essence and mix well. Add milk mixture and knead into soft dough.
Dust the worktop with some flour, place the dough and stretch. Cover the dough with cling film and refrigerate for 20 minutes.
Dust the worktop with some flour and roll out the dough into a thin rectangular sheet. Spread some butter over the sheet, make an E-fold and cover with cling film. Refrigerate for 10-15 minutes. Repeat the process for 5 times.
Dust the worktop with some flour, place the dough on it and roll out to make 1 inch thick sheet. Cut into roundels using a medium size cookie cutter. Create a hole in center using a small cookie cutter. Place the roundels on the baking tray and set aside in a warm place for 10 minutes.
Heat sufficient oil in a pan. Deep-fry the roundels till golden. Drain on absorbent paper.
Take icing sugar in a bowl. Add a few drops vanilla essence and 2-3 tablespoons milk and mix well.
Coat one side of some cronuts in melted dark chocolate. Place all cronuts on the baking tray. Drizzle some of the icing sugar mixture over remaining cronuts. Roughly chop hazelnuts and put some over the chocolate coated cronuts.