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Preheat the oven to 350°F. Line an 8 by 8-inch baking pan with foil, leaving an overhang on all sides.
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In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
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In a large bowl, use an electric mixer to beat the butter, cream cheese, and sugar on medium-high speed until well combined and smooth, about 3 minutes. Add the egg, egg yolk, and vanilla and beat until combined. On low speed, gradually add the flour mixture and beat until combined.
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Press the dough into the prepared pan. Bake for 25 minutes, or until lightly golden brown on top. Place the pan on a wire rack and let cool completely.