1. Heat oven to 180ºC . Grease a 33 cm x 23 cm baking pan. Line with foil. Grease foil.
2. Melt 1 cup chocolate chips in a small saucepan over low heat, stirring often. Remove from heat and cool slightly.
3. Mix flour and baking powder. In a large bowl, beat butter and sugar with electric mixer until fluffy. Beat in eggs; one at a time, beating well after each addition. Beat in vanilla. Stir in flour mixture until blended. Scoop half into a second bowl.
4. Stir melted chocolate into remaining batter until blended. Spread chocolate batter in prepared pan. Drop tablespoonfuls of the plain batter over chocolate layer. Spread carefully in an even layer. Sprinkle with remaining half cup chocolate chips and the walnuts.
5. Bake 30 to 35 minutes until edges begin to pull away from sides of pan.
6. Cool in pan on wire rack before cutting in bars. Makes 24 bars.