Preheat oven to 375 degrees (F).
Whisk flour, 2 tablespoons dark brown sugar, baking powder, baking soda, and salt in large bowl to blend.
Add butter and rub in with fingertips until mixture resembles coarse meal. Mix in the walnuts.
Stir 1/2 cup whipping cream and 1/2 cup maple syrup in small bowl to blend.
Gradually add cream mixture to flour mixture, stirring just until dough comes together and adding more cream by tablespoonfuls if dough is dry. Turn dough out onto lightly floured surface. Knead dough gently until smooth, about 5 turns.
Using floured hands, pat out dough to form an 8-inch round; cut dough into 8 wedges. Transfer wedges to baking sheet, spacing 2 inches apart.
Bake scones for about 20 minutes, until golden and toothpick inserted into center comes out clean. Transfer to rack.
Meanwhile, whisk remaining 2 tablespoons dark brown sugar, 6 tablespoons whipping cream, and 2 tablespoons maple syrup in medium bowl to blend. Gradually whisk in enough powdered sugar to form thick glaze.
Drizzle or spread glaze over warm scones. Sprinkle with additional chopped walnuts if desired. Let stand until glaze sets.