Prep the air fryer and pan: Preheat the air fryer to 320°F (160°C) for 3–5 minutes. Lightly grease a mini muffin pan or silicone cups. If using paper liners, give them a mist of spray to prevent sticking.

Melt the butter: In a small saucepan or microwave-safe bowl, melt the butter until just liquid and warm, not sizzling. Whisk in the granulated sugar and brown sugar until glossy and slightly thick.

Add wet ingredients: Whisk in the egg and vanilla until the mixture looks smooth and lightened. This helps with a shiny top and chewy bite.

Combine dry ingredients: In a separate bowl, whisk together flour, cocoa powder, salt, and baking powder to break up any lumps.

Make the batter: Fold the dry mixture into the wet mixture with a spatula until just combined. Do not overmix. If using chocolate chips, stir them in now.

Fill the cups: Spoon batter into the mini cups, filling each about 3/4 full. This allows room for a slight rise without overflowing.

Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 7–10 minutes, depending on your air fryer model. Start checking at 7 minutes. The tops should look set with a slight sheen; a toothpick should come out with moist crumbs, not wet batter.

Cool briefly: Let the brownie bites rest in the pan for 5 minutes to firm up. Then transfer to a rack to cool completely (or enjoy warm if you like melty centers).

Finish and serve: Optional: sprinkle with flaky salt, dust with cocoa or powdered sugar, or drizzle with melted chocolate. Serve plain or with a scoop of ice cream.