In a large bowl, combine the all-purpose flour, brown sugar, salt, baking soda, and baking powder.
Cut the butter into small pieces and add to the bowl. Using your fingertips, blend in the butter until the mixture resembles coarse crumbs.
Add the buttermilk, maple syrup, and 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 maple syrup and continue to bake until golden, 5 to 10 minutes more.