Use spoon-and-level method to measure flour for best texture., Melt butter carefully without boiling., Remove pan from heat before adding eggs to avoid scrambling, stirring continuously., Fold dry ingredients gently, just until combined to maintain fudgy texture., Chop pecans evenly to avoid burnt spots., Check for doneness at 35 minutes; edges should be firm with moist crumbs on toothpick., Cool brownies fully before slicing; chilling makes cleaner cuts., Warm knife under hot water and dry between cuts for neat slices., Line pan with parchment overhang to lift brownies easily., Flavor variations include swapping nuts, adding bourbon to topping, caramel swirls, extra chocolate, espresso powder, or making nut-free versions with dried fruit or extra chips., Store baked brownies at room temperature for up to 3 days, refrigerated for up to 1 week, or frozen up to 3 months., Reheat individual squares in microwave (10-20 seconds) or oven (300°F for 8-12 minutes)., You can bake brownie layer ahead and bake topping separately, but baking together yields best texture.