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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
Rub lemon zest into brown sugar with fingertips until no zest strands remain. Add flour and combine in medium bowl. Using two dinner knives, cut in butter until mixture is resembles wet sand. Stir in nuts and thyme; set aside.
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3
Combine flour, sugar, baking soda, and salt in large bowl. Add butter and beat on medium speed until combined, about 1 minute (mixture will be crumbly).
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4
In small bowl, beat together eggs, buttermilk, lemon juice, 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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5
Scrape batter into prepared baking pan. Place 12 spoonfuls of preserves on top of batter. Run a knife through cake batter to create a marble swirl. Sprinkle evenly with sugar-walnut mixture.
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6
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. Sprinkle cake with additional thyme leaves, if desired. Serve.
