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Preheat oven to 375 degrees. Butter a baking sheet and set aside.
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In a medium bowl, whisk together flour, baking powder, baking soda, and salt; set aside.
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In the bowl of an electric mixer fitted with the paddle attachment, cream together sugar and butter. Add egg, sour cream, and vanilla, and mix until well combined. With the mixer on low, gradually add enough of the flour mixture to form a soft dough. You may have up to 1/2 cup flour mixture remaining. Transfer dough to refrigerator until chilled.
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Place heaping teaspoons dough on baking sheet and sprinkle with sanding sugar. Transfer to oven and bake until light golden, 8 to 10 minutes.
