Preheat oven to 350°F. Butter a 2-quart oval casserole dish. In a medium bowl, whisk flour, sugar, baking powder and salt. Whisk in 3 Tbsp. cocoa and 1/2 cup walnuts. Add buttermilk and melted butter, stirring to form a thick batter. Spread batter evenly into prepared pan.
In small bowl, combine remaining cocoa, brown sugar and cinnamon. Sprinkle evenly over batter; top with remaining walnuts.
Pour boiling water in a thin stream over entire surface of brownie; do not stir. Bake until top is set and a bit crispy, 40 to 45 minutes. Serve with whipped cream or ice cream, if desired.