Preheat oven to 350 degrees F. Sift the flour, sugar, baking powder, salt and 2 Tbsp. cocoa into a bowl. Mix in the pecans. Pour in milk, melted butter and vanilla and mix until just combined and very thick batter forms.
Spoon the batter into an 8” x 8” non-stick baking pan. Combine the brown sugar and 1/4 cup cocoa powder in another bowl. Sprinkle this mixture over batter. Pour the hot water all over the batter and brown sugar/cocoa mixture. Bake in the centre of the oven 45 minutes, until brownie rises to the top and the pudding below is bubbling. Serve hot, if desired, with vanilla ice cream or whipped cream.
Note: Just in case the pudding bubbles over during baking, to catch any drips, set a sided baking sheet, filled with a little water, on the rack below it the oven.