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Position the oven rack in the middle position; preheat oven to 375°; line a 9 x 13 inch jelly roll pan with foil, shiny side up; coat the foil with butter or vegetable spray.
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Sift the flour, baking powder, and salt together.
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Cream butter and brown sugar in the bowl of a standing mixer fitted with the paddle attachment.
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Add in sour cream, vanilla, and eggs and beat until smooth.
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Add sifted dry ingredients and combine.
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Divide the batter in half.
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Add peanut butter to half of batter and chocolate chips to other half; mix in thoroughly.
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Spread chocolate batter in pan using the offset spatula (layer will be very thin).
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Gently spread the peanut butter batter on top.
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Bake 18 minutes or until a pick comes out clean.
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Topping: remove pan from oven; place on a rack, and sprinkle chocolate chips over the top.
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Allow 5 minutes for chocolate chips to soften and melt.
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Spread chocolate with a spatula.
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Cool on rack 20 minutes more and then refrigerate 1 hour, or until cold and firm.
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Cut into squares or triangles while cold.
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Store between sheets of wax paper in a covered tin in the refrigerator.