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Line 2 large baking sheets (18 x 13-inch) with parchment paper, set aside.
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In a large bowl, sift the flour and baking powder, mix in the sugar.
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Add the butter, egg yolks, sour cream and vanilla extract and use your hands to mix the ingredients together until there are no lumps of butter and a soft, smooth dough is formed.
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Divide the dough into two parts, and roll one part into ¼-inch thick.
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Use a 3-inch circle cookie cutter to cut the dough and dollop 1 teaspoon of Nutella filling at the center of each cookie.
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Brush the edges of the cookies with the egg white, then use your pointer finger and thumb to pinch the dough in three corners creating a triangle.
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Place the cookies on the baking sheet and preheat the oven to 350oF (180oC). Place the pan in the refrigerator or freezer while the oven is preheating.
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Bake for 12-14 minutes or until the cookies are puffed and the bottom is golden brown. Remove from the oven and allow to cool before dusting with powdered sugar and serving.