Spray pan with nonstick vegetable oil cooking spray. Set aside. Mix wheat bran and 1/2 tablespoon soy flour together and sprinkle evenly over the inside of the greased pan, also coating the sides.
Place the unsweetened chocolate and butter in a steel bowl and melt over a saucepan of simmering water. Whisk in 1 cup sugar substitute and 1/4 cup heavy cream. Once thoroughly blended, turn off heat and keep warm until needed.
With an electric mixer on high, beat the eggs, 1 cup sugar substitute and the vanilla extract just until blended. Reduce to low speed and then blend in the chocolate mixture.
With a wooden spoon, mix in the baking powder, 1 1/4 cups soy flour, walnuts, and 1/4 cup heavy cream. Spread evenly into the prepared pan and bake on the center rack for 35 to 40 minutes, or until a toothpick inserted in the center comes out clean. (Do not over-bake or brownies will be dry and hard.) Cool completely before frosting with low carb chocolate frosting. Cut into 5 rows by 5 rows to make 25 pieces.
Low Carb Chocolate Frosting:
Place all ingredients in a medium bowl and whisk until well combined. Use to frost brownies, but make sure brownies are completely cooled, or the butter in the frosting will melt.