Melt 1 cup of the chocolate chips in a small saucepan over low heat, let cool slightly- (Or place the chips in a bowl and microwave on HIGH, stirring every 30 seconds, until melted, about 2 minutes.) Set aside.
Sift together the flour and baking powder and set aside.
In a large bowl, beat the butter and the brown sugar until light and fluffy.
Beat in the eggs, one at a time, beating thoroughly after each addition.
Add in the vanilla.
Stir in the sifted dry ingredients until blended.
Transfer half of the mixture to another bowl.
Stir melted chocolate into one portion until blended.
Spread chocolate batter in the prepared pan.
Drop remaining batter by tablespoonfuls onto the chocolate batter.
Spread carefully into an even layer.
Sprinkle with remaining 1/2 cup chocolate chips and the walnuts.
Bake on the center rack for 30 minutes, or until the edges begin to pull from the sides of the pan.
Cool in the pan on a wire rack before cutting into bars.