Preheat oven to 350 degrees. Butter a 6 to 8 inch square baking pan. Sift the flour with the baking soda and salt onto a sheet of waxed paper. Scrape the vanilla beans and reserve the pods and scrapings. Place the vanilla and the butter in a small saucepan and heat until melted. Beat the egg with the brown sugar at medium high speed until it resembles very loose meringue, about 3 minutes, then add melted butter. Reduce the speed, then add the flour and mix until not quite incorporated. Add the nuts and chopped chocolate; fold until well blended. Transfer the batter to the prepared pan. Bake until the top is dry and golden and a toothpick inserted in the center emerges nearly dry, about 30 minutes. Makes 15 bars.