Preheat the oven to 350 degrees F. Lightly butter a 9 by 13?inch glass baking dish, reserving butter wrapper; set aside. Sift together the flour, baking powder, and salt. Set aside.
Combine the butter and sugar in a medium bowl. Using a wooden spoon, beat the butter and sugar until light and fluffy, about 2 minutes. Add vanilla and the eggs 1 at a time, mixing well after each addition. Add sifted flour mixture, and mix until just combined. Stir in nuts and chocolate.
Place the dough in the prepared baking dish. Using the reserved butter wrapper, spread dough evenly. Bake until golden brown, and a cake tester inserted into the center comes out clean, about 30 minutes. Remove to rack to cool slightly, about 20 minutes. Using a serrated knife, cut into 18 (2 1/4 by 3-inch) bars. Store in an airtight container up to 3 days.
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