Heat a Belgian-style waffle iron. In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Stir in buttermilk, melted butter, and eggs until smooth. The batter will be thick.
Spray waffle iron with cooking spray. Ladle about 1/4 cup batter into each section of the waffle grid; spread batter almost to the edges. Close lid; bake 3 minutes, until no steam emerges from waffle iron.
Transfer cooked waffles to a baking sheet; place in an oven set to low heat, about 200 degrees, while using the remaining batter.
Meanwhile, in a large skillet over medium heat, melt 2 tablespoons brown-sugar butter. Add bananas, and saute until softened and golden brown, 1 to 1 1/2 minutes. Add nuts. Remove skillet from heat, carefully add Grand Marnier, and ignite. Allow alcohol to burn off. Continue to saute until bananas and nuts are well coated.
Split waffles in half. Place a half on each of two plates. Spoon one-quarter of the banana mixture over each waffle half. Top with remaining waffle halves. Spoon another one-quarter of the banana mixture over waffles. Dust each with confectioners' sugar and then drizzle with chocolate.