Heat oven to 180C/160C fan/gas 4 and line 2 baking sheets with baking parchment. Put the lactose free 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 gluten free 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 lactose free 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 lactose free 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.