In a large bowl, stir together chocolate pieces, buckwheat flour, sugar, nonfat dry milk powder, cocoa powder, cinnamon, baking soda and salt. Make a well in the center of the flour mixture.
In a medium bowl, stir together egg whites, buttermilk and vanilla. Add all at once to chocolate mixture and stir until combined.
Lightly coat an 8- to 9-inch cast-iron skillet with nonstick cooking spray. (Or nest two 8-inch square or round disposable foil pans together to make a double thick layer; lightly coat with nonstick cooking spray.) Spoon batter into prepared skillet or foil pan. Arrange banana slices on top.
Arrange medium coals around the edges of the grill. Test for medium-low heat in the center of the grill (not over coals). Place pan on grill rack in the center of the grill (not over coals). Cover and grill for 25 to 30 minutes or until edges start to pull away from the sides of the skillet or pan. Cool for 30 minutes; serve warm. If desired, sprinkle with powdered sugar and serve with strawberries and whipped topping.
Tips: If using sugar Substitute, choose from Splenda® Sugar Blend for Baking or Equal® Sugar Lite. Follow package directions to use product amount equivalent to ⅔ cup sugar. Nutrition Per Serving with Substitute: same as below, except 151 cal., 25 g carb. Exchanges: 1½ other carbo. Carb choices: 1½
Variation: Lightly coat a 6-inch springform pan or an 8- to 9-inch cast iron skillet with nonstick cooking spray. Spread batter in pan; arrange banana slices on top. Brush banana slices lightly with lemon juice. Bake in a 350 oF oven for 65 to 75 minutes for the springform pan or 35 to 40 minutes for the skillet or until a toothpick inserted in center comes out clean and edges start to pull away from the sides of the pan. Cool springform pan on wire rack for 10 minutes; loosen sides of pan and cool 30 minutes before removing sides of pan. For skillet, cool on wire rack 30 minutes before serving. If desired, melt 1-ounce white baking chocolate and ¼ teaspoon shortening in a small heavy saucepan over low heat just until melted; drizzle atop brownie. Or, sprinkle top of brownie with powdered sugar if desired. Serve warm.