These are the blondies you'll become known for. A few notes: The key to their flavor is the heavy dose of vanilla -- don't skimp, and don't use artificial extract. Err on the side of under-baking. These are delicious cold from the freezer or fridge (and easiest to slice that way). Adapted slightly from Cook's Illustrated (July 2005). —Genius Recipes
Adjust oven rack to middle position; heat oven to 350° F. Spread nuts on large rimmed baking sheet and bake until deep golden brown, 10 to 15 minutes. Transfer nuts to cutting board to cool; chop coarsely and set aside.
While nuts toast, line a 13 by 9-inch baking pan with foil, leaving enough overhang to help you lift the blondies from the pan when they're done. [Here are Cook's Illustrated's measurements, but you can just wing it: Cut 18-inch length foil and fold lengthwise to 8-inch width. Fit foil into length of 13 by 9-inch baking pan, pushing it into corners and up sides of pan; allow excess to overhang pan edges. Cut 14-inch length foil and fit into width of baking pan in same manner, perpendicular to first sheet (if using extra-wide foil, fold second sheet lengthwise to 12-inch width).] Grease foil-lined pan with butter or oil.
Whisk flour, baking powder, and salt together in medium bowl; set aside.
Whisk melted butter and brown sugar together in medium bowl until combined. Add eggs and vanilla and mix well. Using rubber spatula, fold dry ingredients into egg mixture until just combined; do not overmix. Fold in chocolate and nuts and turn batter into prepared pan, smoothing top with rubber spatula.
Bake until top is shiny, cracked, and light golden brown, 22 to 25 minutes; do not overbake. Cool on wire rack to room temperature. Remove bars from pan by lifting foil overhang and transfer to cutting board. Cut into 2-inch squares and serve.