Prep the ramekin: Lightly grease your ramekin with nonstick spray or butter. This helps the brownie release and keeps the edges from sticking.

Preheat the air fryer: Set it to 320°F (160°C) for 3 minutes. A quick preheat promotes even cooking and that classic crinkly top.

Melt the butter: In a microwave-safe bowl, melt the butter in short bursts until just liquefied. Let it cool for 1 minute so it doesn’t scramble the yolk.

Whisk in sugars and vanilla: Add granulated sugar, brown sugar, and vanilla to the warm butter. Stir until shiny and combined.

Add the egg yolk: Whisk in the yolk until smooth. The yolk gives structure and that rich, fudgy texture.

Stir in dry ingredients: Add flour, cocoa powder, and salt. Fold gently until no dry streaks remain. Don’t overmix—just combine.

Fold in mix-ins: If using, add chocolate chips and nuts. Save a few chips for the top if you like a glossy finish.

Fill the ramekin: Scrape the batter into the prepared ramekin. Smooth the top with the back of a spoon and sprinkle on any reserved chips.

Air fry: Place the ramekin in the air fryer basket. Cook at 320°F (160°C) for 8–11 minutes, depending on your air fryer and how fudgy you like it. Start checking at 8 minutes.

Check for doneness: The edges should look set and slightly pull from the sides; the center will still look soft and a bit glossy. A toothpick should come out with moist crumbs, not wet batter.

Cool briefly: Let it rest for 3–5 minutes. The carryover heat sets the middle just enough while keeping it gooey.

Serve: Enjoy straight from the ramekin, or top with a scoop of vanilla ice cream, a drizzle of peanut butter, or a few fresh berries.