Heat oven to 350°. Line a 9x13-inch metal baking pan with foil, letting excess hand over edges. Grease foil lightly.
In a medium bowl, combine flour, cocoa, and salt, and set aside.
In a medium saucepan over medium heat, melt butter and chocolate, stirring until smooth. Let cool slightly; then stir in sugar and vanilla. Whisk in eggs 1 at a time until well blended. Stir in flour mixture with a wooden spoon or spatula just until blended. Stir in nuts if you like.
Pour ⅔ of the batter into prepared pan, spreading batter into corners. Spread preserves in an even layer. (Pop it in your microwave for 30 seconds and give it a good stir to loosen it up.) Pour remaining brownie batter in an even layer on top.
Bake 22 to 25 minutes, or until just a few moist crumbs cling to a tester inserted into the center. (Don’t overbake.) Let cool in pan on a wire rack. Remove foil from pan and transfer to cutting board. Serve immediately, or wrap completely in foil and store at room temperature, up to 2 days. (To keep longer, wrap in plastic, then in foil or airtight container, and freeze up to 2 months.) Cut into squares.