Sift flour, 3/4 cup white sugar, 2 tablespoons cocoa powder, baking powder, and salt together in a bowl. Stir milk, butter, and vanilla extract into flour mixture until batter is just combined; fold in pecans. Pour batter into an 8x8-inch baking dish.
Mix 1/2 cup white sugar, brown sugar, and 2 tablespoons cocoa powder together in a bowl; sprinkle over batter. Pour hot water over batter and sugar layer.
Bake in the preheated oven until top cake layer is floating on bottom pudding layer, about 40 minutes.