In a medium bowl, stir together 1/2 cup milk, 1 1/2 tbsp melted butter, egg yolk, vanilla, and 2 tbsp hazelnut liqueur.
Whisk 1 tbsp cocoa into liquid until completely incorporated.
Next, whisk in 2 tbsp confectioners' sugar until completely incorporated.
Then gradually whisk in flour until completely incorporated.
Set aside.
Melt 1/2 tbsp butter in a saucepan over low heat.
Stir 1 tbsp milk and 2 tbs hazelnut liqueur into melted butter.
Stir in 1 tbsp cocoa and 2 tbsp confectioners' sugar.
Set over very low heat to keep warm.
Spray a non-stick frying pan or crepe pan with cooking spray, and heat over medium heat.
Pour about 1/4 cup of batter onto the pan, and swirl to form a very thin disk; cook for about 2 minutes.
Flip, and cook about 1 minute more and then place crepe on a plate.
Add 1/4 sliced bananas to crepe, and spoon 1/4 of the chocolate sauce over the bananas.
Roll or fold crepe, and sprinkle with confectioners' sugar.
Repeat steps 3 and 4.
Serve crepes warm.