Prep the pan and oven. Heat oven to 350°F (175°C).
Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.
Toast the pecans. Spread pecans on a baking sheet and toast for 6–8 minutes until fragrant. Cool, then roughly chop.
This step adds big flavor.
Make the brownie batter. Melt butter in a medium saucepan over low heat. Remove from heat and whisk in sugar. Whisk in eggs one at a time, then vanilla.
Sift in cocoa, flour, salt, and baking powder. Fold until just combined. Stir in chocolate chips if using.
Par-bake the base. Spread brownie batter evenly in the pan.
Bake for 15–18 minutes until the top is set and edges look slightly puffed. The center should still be soft; you’re not fully baking it yet.
Mix the pecan topping. In a bowl, whisk corn syrup, brown sugar, eggs, melted butter, vanilla, and salt until smooth. Fold in toasted pecans.
Layer and return to oven. Pour the pecan mixture evenly over the par-baked brownies.
If needed, use a spatula to nudge pecans into an even layer.
Finish baking. Bake 25–30 minutes more. The topping should be set and shiny, with small bubbles around the edges. A slight jiggle in the very center is fine; it will firm up as it cools.
Cool completely. Let the pan cool on a rack until room temperature, then chill for 1–2 hours for cleaner slices.
Lift out using parchment and cut with a sharp knife, wiping the blade between cuts.