Line the air fryer basket with parchment paper and pour about 2 tbsp melted butter onto the parchment, brushing it evenly over the surface.
Slice the apples thinly and arrange them in an even layer in the basket.
Sprinkle the apples with a little cinnamon and sugar.
Air fry at 175°C / 350°F for about 10 minutes, until the apples are slightly softened.
In a mixing bowl, whisk the eggs with a pinch of salt.
Add the ¾ cup sugar and whisk until combined, then pour in the remaining 7 tbsp melted butter and whisk again.
Add the flour and baking powder and gently mix until no dry flour remains, then stir in 1 tsp cinnamon.
Pour the batter over the softened apples.
Air fry at 160°C / 320°F for about 25 minutes, or until a toothpick inserted into the centre comes out clean.
Let the cake cool slightly, then dust with powdered sugar and serve.