Preheat your oven to 350°F (175°C).
Grease and flour a 9x9 inch baking pan.
In a large bowl, use a hand mixer or a whisk to cream the softened butter, granulated sugar, and cocoa powder together until light and well blended.
In a separate small bowl, briefly stir the vanilla extract into the egg substitute.
Pour this into the butter mixture and beat until just combined.
In another bowl, whisk together the all-purpose flour, baking powder, and salt.
Gradually add the dry to the wet , mixing on low speed or by hand until no flour streaks remain.
Do not overmix.
Gently fold in the chopped walnuts, if using.
Spread the batter evenly into your prepared pan.
Bake for 20–25 minutes. A toothpick inserted into the center should come out with moist, fudgy crumbs attached, not wet batter.
Let the brownies cool completely in the pan on a wire rack before cutting into 20 bars.
