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1
Adjust oven rack to middle position and preheat oven to 350u00b0F. Lightly spray 13- by 9-inch cake pan with baking spray. Line pan with parchment paper or heavy-duty foil lengthwise with one sheet, then crosswise with a second sheet to create a sling; spray once again with baking spray.
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2
Combine flour and brown sugar in medium bowl. Using two dinner knives, cut in butter until mixture is resembles wet sand. Stir in nuts; set aside.
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3
Place chocolate in small bowl and microwave until smooth and melted, 1 to 2 minutes, stopping microwave to stir chocolate every 30 seconds. Set aside and allow to cool.
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4
Sift flour, sugar, baking soda, and salt into large bowl. Add butter and beat on medium speed until combined, about 1 minute (mixture will be crumbly).
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5
In small bowl, beat together eggs, buttermilk, and vanilla. With mixer on medium-low speed, add egg mixture in slow steady stream. Beat just until combined. Pause mixer, scrape sides and bottom of bowl, then beat mixture on medium-high speed until thoroughly combined, about 30 seconds.
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6
Transfer 1 cup batter to medium bowl. Stir in melted and cooled chocolate.
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7
Scrape plain batter into prepared baking pan. Dollop chocolate batter on top of batter.
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8
Run a knife through cake batter to create a marble swirl. Sprinkle evenly with sugar-walnut mixture.
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9
Bake until a toothpick inserted in center of cake comes out clean, 25 to 30 minutes. Transfer cake to cooling rack and cool in pan 10 minutes. Pull cake out onto a cooling rack using the parchment sling and cool completely, about 1 hour. Serve.
