Preheat oven to 350 degrees F. Line a 13x9 inch pan with nonstick foil.
In a 3 quart saucepan, melt butter over medium heat. Stir in sugar and cook gently, stirring constantly, for about 1 minute or until sugar melts and mixture takes on a nice sheen.
Remove from heat and stir in the cocoa powder until smooth. Stir in the salt and baking powder.
Let cool for about 5 minutes, then stir in the salt and baking powder. Stir in the eggs (one by one), then add the vanilla, flour and chocolate chips (make sure batter is cool before adding chips).
Spread batter in pan and bake for 25 minutes or until brownies are set. Let brownies cool on a rack for at least a half hour.
Prepare filling. Beat together butter, salt (if using), both sugars, milk and vanilla. Add flour and stir until flour is incorporated. Spread this mixture over cooled brownies and place in refrigerator for about 30 minutes to help firm up dough. Alternative, you can press the mixture into a 13x9 inch rectangle between two sheets of plastic wrap, then carefully invert it onto the brownies.
In a microwave-safe bowl or in a double boiler, melt chocolate and butter. If using a microwave, combine chocolate and butter and microwave on high for 30 seconds. Stir and repeat until melted. Spread melted chocolate mixture over cookie dough. Sprinkle walnuts over top. Allow chocolate to set, then lift from pan and cut into bars.