Preheat oven to 325°F. Grease 8" square baking pan.
Melt butter and 1/4 cup chocolate chips in double boiler (or for 1 minute in microwave). Combine melted chocolate with sugar. In separate bowl, sift together flour, baking powder and salt. Add to chocolate mixture. Mix in vanilla and eggs. Blend thoroughly and stir in remaining chocolate chips and walnuts. Pour batter into greased pan.
Bake for 30-35 minutes or until center is set (do not overcook). Cool completely before cutting. Brownies should be moist and chewy.