To make the batter: in a medium bowl, stir together the milk, melted butter, egg yolk, vanilla and 2 teaspoons of liqueur. Beat 1 tablespoon cocoa into liquid then mix in the icing sugar, Gradually beat in the flour until completely incorporated. Set aside.
To make the chocolate sauce: melt the butter in a saucepan over low heat. Stir the milk and the hazelnut liqueur into melted butter. Add the cocoa and icing sugar and stir until blended. Keep warm over a very low heat.
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.
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 icing sugar. Repeat steps 3 and 4. Serve crepes warm.