Heat oven to 180C/160C fan/gas 4 and line 2 baking sheets with baking parchment. Put the butter and icing sugar in a large bowl and beat with an electric hand whisk for about 5 mins until pale and fluffy. Add the vanilla extract and beat again until fully incorporated.
Sift in the flour, cornflour and baking powder, and fold into the mixture using a spatula until combined (the dough should have a tacky consistency). Spoon the dough into a piping bag fitted with a large star-shaped nozzle. If all the mixture doesn’t fit, do it in 2 batches.
Pipe swirly circles 5cm diameter onto 2 baking sheets making sure there are 3cm spaces between each swirl.
Bake for 10-12 mins, swapping the trays over halfway through the cooking time so the biscuits are evenly baked, until pale golden and cooked through. Leave to cool on the baking sheets for a few mins, then transfer to wire racks.
While the biscuits cool, make the filling. Put the softened butter in a large mixing bowl and add the icing sugar. Stir together initially with a wooden spoon then switch to electric beaters or a whisk to get the buttercream fluffy and smooth. Add the vanilla extract and beat once more to combine. Transfer the buttercream to a piping bag and snip off the end.
Turn the biscuits over so their flat side is facing up then pipe buttercream over half of the biscuits and spread a little jam on the rest. Sandwich a jam covered biscuit together with a buttercream one and repeat until all the biscuits are used up.