Heat oven to 180C/160C fan. 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, cardamom powder 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 4cm diameter onto the baking parchment making sure there are 3cm spaces between each swirl. Place in the fridge once piped for 20 minutes. TIP: Draw circles on to the back of the baking parchment to make sure they're all the same size!)
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.
Heat up a pan and then take it off the heat and then toss the Saffronly saffron threads in to the pan till it becomes crispy (approx 30-45 seconds). Then crush it using the back of a spoon before using it in the next step. (You can store the extra crushed saffron in an airtight jar and use it in the next recipe)
While the biscuits cool, make the filling. Put the softened butter and ground saffron 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 sandwich a biscuit with a covered biscuit together with a buttercream one and repeat until all the biscuits are used up.
Sift icing sugar over the whirls for a Christmassy finish.
