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Preheat the oven to 350F. Line a 9 x 13 inch pan with parchment paper.
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Cream butter and brown sugar until light and fluffy. Mix in peanut butter, eggs, and vanilla.
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In a separate bowl, combine flour, baking powder, baking soda, and salt. Add to wet mixture, stirring until it is just combined. Pour into prepared pan and sprinkle chocolate chunks over top.
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Bake for about 25 minutes or until a toothpick inserted into the center comes out clean.