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 tsp. 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 crêpe 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 crêpe on a plate. Add 1/4 sliced bananas to crêpe, and spoon 1/4 of the chocolate sauce over the bananas. Roll or fold crêpe, and sprinkle with confectioners' sugar. Repeat steps 3 and 4. Serve crepes warm.