Prepare tins. Grease two 15cm (6 inch) tins and line the bases. Heat the air fryer to 140°C.
Mix batter. Whisk the sponge ingredients, apart from the blueberries, until just combined.
Layer berries. Spoon half the batter into the tins and scatter over half the blueberries. Add the rest of the batter, then the remaining blueberries.
Bake cakes. Air fry at 140°C for 25 to 30 minutes until risen, golden and a skewer comes out clean. Bake in batches if the tins don't fit together.
Cool cakes. Leave in the tins for 10 to 15 minutes, then cool completely on a wire rack. Trim the tops level if needed.
Make buttercream. Beat the butter until creamy. Gradually beat in the icing sugar and vanilla. Loosen with a little milk.
Fill cake. Spread 150 g buttercream on the first layer, make a well and add 2 tbsp jam. Top with the second cake.
Cover cake. Spread the remaining buttercream over the top and sides.
Swirl jam. Dot the last tbsp of jam over the cake. Drag a toothed scraper round the sides and swirl the top.
