Prep the oven and pan. Heat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats to prevent sticking.

Mix the wet ingredients. In a medium bowl, whisk the egg, egg yolk, oil, water (start with 1 tablespoon), and vanilla until smooth and glossy.

Add the brownie mix. Pour the dry brownie mix into the bowl. Stir with a spatula or wooden spoon until just combined. The dough should be thick, like cookie dough. If it’s crumbly, add another 1 teaspoon of water at a time until it holds together.

Fold in extras. Stir in chocolate chips or chunks if using. Nuts, toffee bits, or white chocolate also work well here.

Scoop the dough. Use a 1.5-tablespoon cookie scoop (or rounded tablespoon) and place mounds 2 inches apart on the prepared sheets. For thicker cookies, chill the scooped dough for 10 minutes.

Bake. Bake for 8–10 minutes, until the edges look set and the tops are slightly crackly. The centers may look a touch soft—that’s perfect.

Finish and cool. If you like, sprinkle a pinch of flaky sea salt on top right after baking. Let cookies cool on the sheet for 5 minutes, then transfer to a rack to finish cooling.

Enjoy. Serve warm for maximum fudginess, or let them cool fully for a chewier bite.