Prepare a small baking pan (mine was a 18cm (7") springform) by greasing and using baking paper on the bottom of the pan.
Using an electric mixer or food processor, cream the butter and sugar until light and fluffy (around 3-4 minutes). Scrape down the sides of the bowl.
Whisk the egg in, until well incorporated.
Combine the flour, baking powder and baking soda and then add to the wet ingredients. Stir just until combined.
Whisk together the milk and lemon juice and pour in. Don't over mix, just stir until everything is combined.
Pour into the prepared pan and place it in your air fryer on 155℃ (311℉) for around 25 minutes. Check every now and again to make sure it's not browning too quickly on top. The cake is ready when an inserted skewer comes out clean.
Allow to cool in the pan for around ten minutes, then remove the outer edge (if using a springform). After about 30 minutes, turn the cake out onto a wire rack to cool completely.
Sprinkle with icing sugar, a little lemon zest, and cut the lemon into slices and arrange on the cake for decoration (optional).