Prep the pan. Heat oven to 350°F (175°C). Line an 8x8-inch metal pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Melt the butter. In a medium saucepan or microwave-safe bowl, melt 10 tablespoons unsalted butter until just liquid, not sizzling. Let cool 3–4 minutes so it’s warm, not hot.

Whisk in sugars. Add 1 cup packed light brown sugar and 1/4 cup granulated sugar. Whisk until glossy and combined. The mixture should look smooth and syrupy.

Add eggs and vanilla. Whisk in 2 large eggs, one at a time, then 2 teaspoons vanilla. Whisk for 30–45 seconds until slightly thickened. This helps create that signature shiny top.

Combine dry ingredients. In a separate bowl, stir together 1 cup all-purpose flour, 1 tablespoon cornstarch, 1/2 teaspoon baking powder, and 1/2 teaspoon kosher salt.

Fold gently. Add dry ingredients to the wet mixture. Switch to a spatula and fold just until no dry streaks remain. Avoid overmixing.

Optional add-ins. Fold in 1/2 to 3/4 cup chocolate chips or chunks, and/or 1/2 cup chopped toasted nuts. Keep total add-ins to about 1 cup.

Pan and smooth. Scrape batter into the prepared pan and spread evenly. If using, sprinkle with a small pinch of flaky sea salt.

Bake. Bake 22–28 minutes. The edges should be set and deep golden, and the center should look just set with a slight jiggle. A toothpick should come out with moist crumbs, not wet batter.

Cool and slice. Cool in the pan on a rack for at least 30–45 minutes. Lift out using parchment and cut into 12–16 bars with a sharp knife. For neat edges, chill 20 minutes before slicing.