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1
Adjust oven rack to middle position and preheat oven to 350u00b0F. Coat a 17 1/4- by 11 1/2-inch jelly roll pan with baking spray.
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2
Whisk flour, baking powder, baking soda, and salt together in medium bowl; reserve. In large bowl, beat butter and brown sugar with an electric mixer on medium speed until light and fluffy, about 2 minutes. Add eggs, one at a time, beating well after each addition, scraping sides and bottom of bowl with rubber spatula as needed. Add vanilla and beat just to incorporate.
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3
Add flour mixture to butter mixture in three additions, alternating with buttermilk. Stir in white chocolate chips and macadamia nuts.
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4
Scrape batter into prepared pan, making sure surface is even. Bake until cake tester inserted in cake comes out clean, 25 to 30 minutes. Transfer to cooling rack and cool completely, about 1 hour.
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5
Combine sugar, corn syrup, and water in medium heavy-bottomed saucepan. Cook over medium-high heat until the sugar is dissolved. Dampen clean pastry brush with water and wipe down any sugar crystals from sides of pan. Boil mixture, swirling (but not stirring) pan occasionally, until sugar turns deep amber, 5 to 7 minutes. Immediately remove from heat and add cream and salt; mixture will bubble and sputter. Return to heat and stir over low heat until mixture is smooth.
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6
Drizzle salted caramel over cooled cake and cut into 16 or 24 pieces; serve.
