Grease 3 6-ounce ramekins and set aside.
Melt the butter and chocolate in a microwave-safe bowl for about 1 minute, stirring every 30 seconds. Let cool slightly.
In a large bowl, beat the egg, egg yolk, vanilla extract, and sugar until well combined.
Add the flour, melted chocolate mixture, and a pinch of salt, stirring until smooth.
Pour the batter evenly into the prepared ramekins, filling each halfway.
Place ramekins in the air fryer basket and cook at 370°F for 7-10 minutes.
Remove the ramekins from the air fryer carefully and let cool for 1 minute.
Use a butter knife to loosen the cake from the ramekin and flip it onto a plate. Serve warm with whipped cream, berries, or ice cream.