Lightly grease a brownie tin that will fit in your air fryer.
Put the flour and baking powder in a bowl and add the rubber. Rub in with fingertips until the mixture resembles breadcrumbs. Stir in the sugar.
Break the egg into a measuring jug, then make up to 150ml (5 fl oz) with milk. Gradually add to the flour (you may not need it all), and mix to a soft dough.
Turn out onto a floured surface and roll out to a thickness of ½ inch (1 cm). Cut into rounds with a fluted cutter and place in the tin.
Turn the air fryer to 200℃ and bake for 6 minutes.
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