Preheat the oven to 350 degrees F. Line an 8-inch square baking pan with foil and coat the foil with fat-free cooking spray.
Whisk together in a small bowl the flour, baking powder, and salt.
In a large bowl, whisk together the sugar substitute, sugar, eggs, and vanilla.
In a medium microwave-safe bowl, combine the margarine and chocolate chips, and microwave on medium heat just until melted, about 30 seconds. Whisk in the cocoa powder, butter replacement, and instant espresso (optional). Stir into the sugar mixture. Stir in the flour mixture and walnuts.
Scrape into the prepared pan and bake until a toothpick inserted into the center comes out with moist crumbs on it, 25 to 30 minutes. Cool in the pan for 5 minutes. Using the foil, lift the brownies out of the pan and cool completely on a rack. Cut into 16 even pieces.