Preheat oven to 350 degrees F.

Lightly butter a 13 x 9-inch baking pan.

Melt 1 cup of the chocolate chips in a small saucepan over low heat; cool slightly. (Or place the chocolate in a heat proof bowl and microwave on High, stirring every 30 seconds, til melted, about 2 minutes.) Set aside.

Sift together the flour and baking powder; set aside.

In a large bowl, beat the butter and the brown sugar with an electric mixer until light and fluffy.

Beat in the eggs, one at a time, beating thoroughly after each addition. Beat in the vanilla.

Stir in the sifted dry ingredients til blended.

Transfer half of the mixture to a second bowl.

Stir the melted chocolate into one portion until blended. Spread the chocolate batter in the prepared pan.

Drop the remaining batter by tablespoonfuls onto the chocolate batter. Spread carefully into an even layer.

Sprinkle with the remaining 1/2 cup chocolate chips and the walnuts.

Bake on the center rack for 30 minutes, or until the edges begin to pull away from the sides of the pan.

Cool in the pan on a wire rack before cutting into bars.