Butter 13x9x2-inch baking pan. Melt butter and chocolate in heavy large saucepan over low heat, stirring constantly until smooth. Remove from heat. Whisk in sugar, then eggs and vanilla. Mix flour, baking powder and salt in small bowl. Add to chocolate mixture and whisk to blend. Stir in nuts. Pour 2 C batter into pan. Freeze until firm, about 10 minutes. Preheat oven to 350'F. Spread preserves over brownie batter in pan. Spoon remaining batter over. Let stand 20 minutes at room temperature to thaw bottom layer. Bake brownies until tester inserted into center comes out clean, about 35 minutes. Transfer to rack and cool. Cut brownies into squares. (Can be prepared 1 day ahead. Store in airtight container at room temperature.) Bon Appetit/August/94 Scanned & edited by Di Pahl & <gg>