Prep the air fryer and liner. Preheat your air fryer to 350°F (175°C) for 3–5 minutes. Cut a piece of parchment to fit the basket, leaving a little space around the edges for air flow. Poke a few holes if it’s not perforated.

Cream the wet ingredients. In a bowl, beat 1/2 cup softened unsalted butter, 1/2 cup granulated sugar, and 1/2 cup light brown sugar until smooth and slightly fluffy, about 1 minute. Stir in 3/4 cup creamy peanut butter, 1 large egg, and 1 teaspoon vanilla extract until combined.

Add the dry ingredients. In a separate bowl, whisk 1 cup all-purpose flour, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Add to the wet mixture and stir just until no dry spots remain. If using mix-ins, fold in up to 1/2 cup now.

Chill briefly (optional but helpful). If the dough feels sticky, chill for 15–20 minutes to make scooping and shaping easier. Chilled dough also holds its shape better in the air fryer.

Portion and shape. Scoop dough into 1 to 1.5 tablespoon balls. Roll gently and place on the parchment-lined basket, spacing them at least an inch apart. Use a fork to press a crosshatch pattern, flattening to about 1/2 inch thick.

Air fry. Cook at 350°F (175°C) for 6–8 minutes. The edges should look set and lightly golden; the centers may look slightly soft. That’s perfect.

Rest to set. Let cookies cool in the basket for 3–5 minutes to firm up before moving to a rack. Sprinkle with flaky sea salt while warm if you like.

Repeat. Continue with remaining dough in small batches. Keep dough chilled between batches for best shape.