To prepare cake batter, combine flour, brown sugar, cocoa, baking powder, and salt in a medium bowl. Set aside.
In a small bowl whisk together milk and oil. Add milk mixture to flour mixture. Using a wooden spoon, stir vigorously until well blended, (you may also use an electric mixer on low speed). Pour batter over the bottom of an ungreased 8x8 baking pan. Spread evenly.
In a small bowl, mix together second amount brown sugar and second amount cocoa. Sprinkle mixture evenly over batter. Pour boiling water over top. DO NOT STIR! Bake for 40-45 minutes, until top of cake feels dry to the touch. Remove from oven and let stand 5 minutes before serving. Cut into 6 pieces. Spoon sauce from bottom of pan over individual servings of cake.