Prepare a small baking pan - mine was an 18cm (7") 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, just until well incorporated.
Combine the flour, baking powder and baking soda and then add to the wet ingredients. Stir just until combined.
Add the instant coffee to the milk and mix together. Leave for a minute or two until the coffee dissolves.
Turn on your Airfryer (about two minutes before the cake is ready to bake).
Add the coffee and milk mixture to the other ingredients. 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 20-25 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.