In a large bowl, combine 2 1/2 cups all-purpose flour, 1 tablespoon brown sugar, 1/2 teaspoon salt, 2 teaspoons baking soda, and 1 tablespoon baking powder. Cut 4 tablespoons chilled unsalted butter into small pieces and add to the bowl. Using your fingertips, blend in the butter until the mixture resembles coarse crumbs. Add 1/2 cup buttermilk, 1/3 cup maple syrup, and 1/2 cup coarsely chopped walnuts and mix with a spoon until the dough comes together. On a lightly floured surface, pat the dough into a 1-inch-thick circle. Use a knife to cut the dough into 8 wedges and bake on a greased baking sheet at 375º F for 15 minutes. Brush the tops with 1 tablespoon maple syrup and continue to bake until golden, 5 to 10 minutes more.