Spray an 8- or 9-inch square baking pan with cooking spray and set it aside. Preheat the oven to 350° F. In a medium-sized bowl, combine the dry ingredients for the cake layer, breaking up any clumps of cocoa powder with the back of a spoon. Combine the milk, melted butter or margarine, and vanilla extract in a measuring cup or small bowl, and add all at once to the dry ingredients. Stir just until blended, then scrape and pour the batter into the prepared baking pan. Smooth the top of the batter as well as you can.

In a small bowl, combine the brown sugar and 1/4 cup cocoa powder, breaking up any lumps with the back of a spoon. Spread this mixture evenly over the cake layer, gently shaking the pan from side to side to distribute it evenly.

On the oven rack, if it slides easily, or on a countertop as near as possible to the oven, pour the boiling water over the cake carefully. Do not stir the mixture! Transfer the cake to the oven (or slide the rack into the oven), using oven mitts or potholders. (The pan will be quite hot.) Bake for 35 to 40 minutes, or until the cake pulls away from the sides of the pan. A toothpick inserted near the center of the cake should come out clean, though the center will be quite jiggly. 

Set aside to cool a little before serving with ice cream or whipped cream. Refrigerate any leftovers, and reheat individual servings on the medium setting of your microwave for about 30 seconds before serving.