In a medium bowl, whisk together eggs, buttermilk and vanilla extract.
In a separate bowl, sift together cinnamon, flour and baking powder. Add sugar. Whisk into buttermilk mixture and stir in melted butter. Allow batter to sit 10-15 minutes before making.
Heat a flat, non-stick pan over medium heat for 5 minutes. Grease lightly with oil or spray and pour about 2 oz of batter into the center of the pan. These crêpes will be thicker than usual crêpes. Cook for about 2-3 minutes, until edges brown. Lift crêpe gently with a spatula and flip over. Cook for 1 minute and remove from heat.
Bring water and sugar up to a full boil and cook for 2 minutes. Remove from heat, stir in hazelnut liqueur and allow to cool.
Heat cream to just below a simmer (on the stove or in the microwave) and pour over chocolate. Let sit for 2 minutes, then stir gently to blend. Allow to completely cool before finishing gateau, or chill until ready to use and warm slightly in microwave.
Place one crêpe on a large plate or platter. Brush lightly with the hazelnut syrup. Using a palette knife, spread a thin layer of ganache over the entire surface of the crêpe. Set another crêpe on top and repeat process using all the crêpes, sparing the syrup and chocolate layer on the top one. Cover with plastic wrap and chill until ready to serve.