Preheat oven to 375 degrees F. Position rack in the center of the oven. Lightly spray an 8-inch square baking dish.
In a small bowl, sift flour and baking powder together; set aside.
In another small bowl, blend the cocoa, water, vanilla extract, and espresso powder together; set aside.
In a large metal bowl set over a pan of hot, not simmering, water (bottom of the bowl should not touch the water), combine the butter and chocolate and let stand, stirring occasionally, until smooth and melted. (The mixture can be melted in the microwave on high power, stirring every 15 seconds. Remove when there are still a few lumps of chocolate and stir until fully melted). Learn different techniques for How To Melt Chocolate.
In a medium-size bowl, combine the beaten eggs, sugar, and Splenda. Stir in the cocoa mixture. Add the melted chocolate/butter mixture and stir to combine. Stir in the flour mixture until just incorporated (do not over mix).
Pour the batter into the prepared pan and smooth the top. Bake until a toothpick inserted into the center comes out with a few moist crumbs attached, 18 to 20 minutes, rotating the pan halfway through baking (do not over bake - the brownies will still look under baked when you take them out of the oven. They will set up as they cool). Remove from oven and let cool completely, approximately 1 hour, on a wire cooling rack before cutting.
Diet Dark Chocolate Brownie Recipe: https://whatscookingamerica.net/desserts/dietdarkchocbrownies.htm