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Sift together the flour, baking power, and salt; set aside.
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Cream the butter and sugar together in a mixer with the paddle attachment on medium until light and fluffy, 2 to 3 minutes.
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On the lowest speed, beat in the flour, scraping down the bowl as necessary. Mix only until the flour is incorporated and the dough is smooth.
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Put the dough into the prepared pan and press it to all the sides. Bake until it is puffed and just beginning to brown, about 15 minutes. Let cool on a wire rack while you make the topping.