Prep the pan and air fryer: Lightly grease a small baking pan or line it with parchment, leaving a little overhang to lift the bars out. Preheat the air fryer to 320°F (160°C) for 3–5 minutes for even baking.

Melt the butter: Melt the butter gently on the stovetop or in the microwave. Let it cool for 2–3 minutes so it doesn’t scramble the egg.

Mix the wet ingredients: In a medium bowl, whisk the melted butter and brown sugar until glossy and smooth, about 30 seconds. Whisk in the egg and vanilla until combined and slightly thick.

Combine the dry ingredients: In a separate small bowl, whisk the flour, baking powder, baking soda, and salt. This prevents clumps and uneven rise.

Make the batter: Add the dry mixture to the wet and stir with a spatula until just combined. The batter will be thick. Fold in the chocolate chips and any optional mix-ins.

Spread evenly: Scrape the batter into your prepared pan and smooth the top. Press a few extra chips on top for a bakery look.

Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 13–17 minutes, depending on pan size and air fryer model. The edges should be set and lightly golden; the center should look slightly underbaked.

Check doneness: Insert a toothpick near the center. You want moist crumbs, not wet batter. If needed, cook in 1–2 minute bursts. Avoid overbaking; carryover heat will finish the center.

Cool and slice: Let the pan cool on a rack for at least 20–30 minutes. Lift out using the parchment and cut into bars. They’ll firm up as they cool but stay soft and chewy.

Serve: Enjoy warm with ice cream, or at room temperature. A pinch of flaky sea salt on top is a nice finishing touch.