In a medium-sized mixing bowl, whisk together the sugar, flour, cocoa, salt, baking powder, and espresso powder. Add the eggs, melted butter, and vanilla, beating till smooth. Scoop the batter into a lightly greased 9-inch microwave-safe pie pan (glass or ceramic), or a microwave-safe 9-inch round or 8-inch square cake pan. Pour the boiling water very gently over the top of the cake; don't stir it in. I know this looks weird, but trust me. Cover the pan with a dinner plate, glass lid, or whatever you usually cover pans with in the microwave. Microwave the cake for 5 minutes at regular power. Remove the cover, and poke a spoon into the center of the cake. It should be set, but have a layer of chocolate "sauce" at the bottom. If it's done, remove it from the oven, and wait 5 minutes before serving. If it seems way too moist, put it back into the microwave, without a cover, and cook for another minute or so. Sprinkle with confectioners' sugar, if desired, before serving. Yield: 1 cake, 8 servings. Nutrition information per serving (80g): 205 cal, 6g fat, 5g protein, 9g complex carbohydrates, 24g sugar, 2g dietary fiber, 88mg cholesterol, 169mg sodium, 406mg potassium, 64RE vitamin A, 3mg iron, 55mg calcium, 116mg phosphorus, 22mg caffeine.