Grease three 6-ounce ramekins and set them aside.
In a large microwave-safe bowl, combine butter and chocolate that has been broken into pieces. Microwave on high power in 30-second intervals, stirring well in between. It usually takes about a 1-minute until chocolate is melted and the mixture is smooth. Set aside.
In a separate large bowl, beat 1 large egg, 1 egg yolk, 1/2 tsp vanilla extract, and 3 Tablespoons sugar together until eggs are thoroughly beat. You may use an electric beater.
Next, add the chocolate mixture, 3 tablespoons of flour into the egg mixture. Stir until combined.
Pour the mixture into the ramekins till about halfway. Place the ramekins into the air fryer basket.
Cook in the air fryer at 380°F for about 8 to 9 minutes.
Once the air fryer lava cake is ready, take the ramekins out of the basket with a clean, thick dish towel. Let the cake cool in the ramekin for about 1 minute.
Use a butter knife to loosen the cake from the ramekin and turn it over onto a plate. Serve with fresh berries, top with powdered sugar or vanilla ice cream.