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Preheat oven to 375°F.
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MAKE THE CRUMBS: Mix together the brown sugar, flour, peanut butter, and melted butter until crumbly; set aside.
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MAKE THE CAKE BATTER: Cream together the peanut butter and butter until light and fluffy.
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Slowly beat in the brown sugar.
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Add the eggs, one at a time, beating until fluffy after each addition.
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Whisk together the flour, baking powder, baking soda, and salt; add alternately to the creamed mixture with the milk, blending well after each addition. (Do not overmix).
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Spread the batter evenly in a greased 13x9-inch baking pan; top with the crumbs.
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Bake at 375°F for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
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Cool slightly before serving.
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Dust with powdered sugar, if desired; or drizzle with a chocolate glaze.