Place the semi-sweet chocolate and unsalted butter in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until fully melted and smooth. Let the mixture cool for about 2 minutes.
Whisk the powdered sugar into the chocolate mixture until smooth. Add the eggs, egg yolks, vanilla extract, and salt, and whisk until everything is well combined and glossy.
Gently fold in the all-purpose flour with a spatula, mixing until just incorporated. Be careful not to overmix - a few small streaks of flour are fine, as they will blend in.
Spray four 6-oz ramekins generously with nonstick cooking spray, making sure to coat the bottoms and sides thoroughly. Divide the batter evenly among the ramekins, filling each about three-quarters full.
Preheat the air fryer to 375 degrees F. Place the ramekins in the air fryer basket, leaving a little space between each one for air circulation. Cook for 8 to 9 minutes, until the tops are set and slightly cracked but the centers still jiggle when gently shaken.
Let the lava cakes rest in the ramekins for exactly 1 minute. Run a thin knife around the edges, then carefully invert each ramekin onto a plate. Tap the bottom gently and lift the ramekin away to reveal the cake. Serve immediately while the molten center is still flowing.