Brown the butter: Add the butter to a light-colored saucepan over medium heat. Let it melt, then bubble and foam, stirring occasionally, until you see golden bits on the bottom and it smells nutty, 5–7 minutes. Immediately pour into a heatproof bowl, scraping in the browned bits. Cool 10–15 minutes.

Prep the pan: Heat the oven to 350°F (175°C). Line a 9x9-inch (23 cm) pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Mix the sugars and butter: Whisk granulated sugar and brown sugar into the warm brown butter until combined and glossy, 30–45 seconds.

Add eggs and flavor: Whisk in eggs, vanilla, and almond extract (if using) until the mixture looks thick and slightly lightened, about 30 seconds.

Combine dry ingredients: In a separate bowl, whisk flour, cornstarch, baking powder, baking soda, and salt.

Bring it together: Switch to a spatula and fold the dry mix into the wet just until no dry streaks remain. The batter will be thick and glossy.

Pan and top: Scrape batter into the pan and smooth the top. Sprinkle with coarse sugar for a sparkly crust.

Bake: Bake 22–28 minutes, until the edges are set and lightly golden and the center looks just barely done. A toothpick should come out with a few moist crumbs, not wet batter.

Cool and slice: Cool in the pan on a rack at least 30–45 minutes. Lift out using the parchment and cut into squares.