Make the Caramel: Add the sugar to a small saucepan over medium heat. Let it melt, stirring occasionally, until it turns a deep amber color. Immediately pour the caramel into your ramekins, swirling to coat the bottoms evenly.
Mix the Custard: Whisk together the eggs, condensed milk, whole milk, vanilla, and salt until smooth. Avoid over-whisking to minimize air bubbles.
Strain: Pour the mixture through a fine-mesh sieve into a measuring cup or bowl for an ultra-silky custard.
Fill & Air Fry: Pour the custard over the caramel, filling each ramekin almost to the top. Cover tightly with foil. Fill the basket with boiling water (should reach 3/4 of the ramekin height). Air fry at 425F for 25-30 mins until the edges are set and the center has a slight jiggle.
Chill: Let the flan cool to room temperature, then refrigerate for at least 4 hours, preferably overnight, to fully set.
Unmold & Serve: Run a knife around the edge of each ramekin. Place a plate on top and invert to release the flan. Serve chilled with the caramel sauce spooned over the top.