Melt the butter and chocolate in a heavy saucepan over low heat, stirring constantly until smooth. Remove from the heat. Whisk in the sugar, eggs and vanilla. Mix the flour, baking powder and salt in a small bowl. Add to the chocolate mixture and whisk to blend. Stir in the nuts.
Pour 2 cups of batter into a buttered 13x9 inch pan (or use the specialty brownie pan shown below for lots of edges). Freeze until firm, about 10 minutes. Spread the preserves over the frozen batter. Spoon the remaining batter over the preserves. Let stand for 20 minutes at room temperature to thaw.
Bake at 350°F for 35 minutes or until tester comes out clean. Transfer to a rack to cool.