Grease and line the base and sides of an air fryer tray with baking parchment.

Whisk together the butter, lemon zest and caster sugar until pale and fluffy. Whisk in the eggs, 1 at a time. Stir the flour and 50g chopped Walnuts into the cake mixture with the juice of ½ lemon. Spoon into the prepared tray, level the surface and bake in the air fryer at 180oC for 35-40 minutes until a skewer comes out clean, cover with foil towards the end if it looks a little dark.. Allow to cool.

Blend the icing sugar with 1-2 tsp lemon juice to give a smooth and thick, but pipeable consistency and drizzle, or pipe over the cake and sprinkle with remaining Walnuts. Cut into 9 to serve.