Set your air fryer to 375°F (190°C) and let it warm for about 3 minutes. Lightly oil the basket or use perforated parchment.
Stir together the flour, baking powder, sugar, and salt In a mixing bowl until well blended.
Grate the ice cold butter on the largest openings on a box grater.
Add the cold grated butter pieces and rub them into the flour using your fingers. Toss to coat the butter with flour.
Work in the cold butter or a pastry cutter. Stop when the mixture looks sandy with small butter chunks still visible.
Pour in the milk and gently mix just until a soft dough forms. The dough should look slightly rough — do not knead it smooth.
Turn the dough onto a lightly floured surface. Pat it into a slab about ¾ inch thick.
Fold the dough over in half.
Flatten it into a rectange again and fold in half.
Flatten the dough again to ¾" in height.
Cut out four biscuits using a cutter or sharp knife. Press straight down so they rise evenly.
Gather the remaining dough and stamp out another biscuit. Do not overwork the dough scraps or they will become tough.
If desired, melt a little butter and brush it on the tops of the biscuits before air frying them.
Place the biscuits in the basket with a little space between them. Air fry at 375°F (190°C) for 8–10 minutes, flipping once halfway through, until puffed and golden.
Let them cool for 2 minutes, then serve warm.