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Preheat oven to 350F and line a 13x9 pan with parchment paper**
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Combine melted butter and sugar in a large bowl and stir well.
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Add eggs, egg yolk, and vanilla extract and stir until completely combined. Set aside.
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In a separate bowl, whisk together flour, cornstarch, baking powder, and salt.
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Gradually stir dry ingredients into wet until completely combined.
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Fold in white chocolate chips and nuts, if using.
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Spread blondie batter into prepared pan and transfer to oven.
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Bake on 350F (175C) for 25-30 minutes*** or until a toothpick inserted in the center comes out clean or with a few fudgy crumbs.
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Allow to cool before cutting and enjoying