Preheat the air fryer to 320°F (160°C) for 3–5 minutes. Preheating helps the muffins rise evenly.

Grease your mini muffin cups. Use a mini muffin tin designed for air fryers or silicone mini cups. Lightly coat with nonstick spray or oil.

Mix dry ingredients. In a medium bowl, whisk together the flour, sugar, baking powder, and salt until well combined.

Mix wet ingredients. In a separate bowl or large measuring cup, whisk the milk, egg, oil (or melted butter), and vanilla until smooth.

Combine gently. Pour the wet mixture into the dry ingredients. Stir with a spatula just until the flour disappears. Do not overmix. The batter should be slightly lumpy.

Add mix-ins. Fold in your choice of chocolate chips, fruit, or nuts. If using frozen blueberries, add them straight from the freezer to prevent bleeding.

Fill the cups. Spoon the batter into the prepared mini cups, filling each about 3/4 full. This gives a nice dome without overflow.

Air fry. Place the muffin tray or silicone cups in the air fryer basket, leaving a bit of space for air circulation. Cook at 320°F (160°C) for 7–10 minutes until the tops are set and lightly golden. A toothpick should come out clean or with a few moist crumbs.

Cool briefly. Let the muffins rest in the cups for 2–3 minutes, then transfer to a rack to cool. This helps them set and keeps the bottoms from steaming.

Repeat with remaining batter. Depending on your air fryer size, you may need to bake in batches.