Grease a bread pan or line it with parchment paper. Combine butter, sugar, eggs, flour, a pinch of salt and two-thirds of the lemon zest into a bowl. Beat with an electric mixer for 4 to 5 minutes until pale and fluffy.
Heat the air fryer to 160 degrees Celsius. Pour the mixture into the tin and smooth the surface. Cook for about an hour, and check to see if it’s done by skewering with a toothpick. If the pick comes out clean, it’s done.
Leave to cool for about 5 minutes. In the meantime, combine the icing sugar with lemon zest and juice to make it a smoothing icing.
Remove the warm cake from the pan and place on a wire rack. Poke deep holes in the top, and drizzle over with the icing. Leave to set before serving.
