Combine the butter and icing sugar in a mixing bowl, and beat with a wooden spoon for 2 minutes. Add the flour, cornflour, vanilla, and salt, and mix until a soft dough forms. Fold through the chocolate chips and
sprinkles. Cover the dough and chill it for a minimum of 1 hour (and up to 1 day).
Prepare the buttercream filling while your dough is chilling. Place the softened butter in a large mixing bowl, and gradually add the sifted icing sugar, beating until fluffy. If you have an electric hand whisk, use this to speed up the process. Add the cream cheese, vanilla, and salt, then beat until combined. Set aside in thefridge.
Using a tablespoon measuring spoon, portion the dough into 24 scoops. Roll them into balls.
Preheat the air fryer to 160°C/325°F for 3 minutes. Line with a silicone mat.
Working in batches, if necessary, place the dough balls into the preheated lined air fryer basket, leaving a 5cm (2in) gap between them, as they will expand. Bake at 160°C/325°F for 12 minutes.
Leave the cookies to cool for 5 minutes before removing them from the air fryer basket with a spatula. Place on a wire rack to cool completely. Repeat the process until all the cookies are baked.
Take two cookies and sandwich them together with a spoonful of the buttercream filling, squeezing them together until the icing just escapes the sides. Roll the sides of each sandwiched cookie in sprinkles to decorate.
These will keep for 3 days in an airtight container in the fridge.