Preheat the oven to 180'C and line two baking sheets with parchment paper. Set aside.

Add the butter and icing sugar to a large mixing bowl and beat with an electric whisk for 3-4 minutes until pale and fluffy. Add the vanilla extract and beat again until combined.

Sift in the flour, xanthan gum cornflour and baking powder. Fold into the butter and sugar mix until fully combined. You want quite a thick dough, but still pipe-able. Spoon the mixture into a large piping bag with a star nozzle.

On the baking sheet, pipe 10 swirly circles. They should be approx 5cm in diameter with approx 3-4cm between them to allow for spreading. To pipe, start in the centre of the circle and swirl out, then swirl back into the middle so it resembles an iced cupcake-type shape.

Bake in the oven for 7-8 minutes (I baked one at a time, if baking at the same time you may need to switch the trays in the oven half way through) until golden on top. Remove from the oven and cool completely on the trays.

To make the filling: Beat the butter, icing sugar and vanilla extract for 2-3 minutes with an electric whisk until smooth. Spoon into a piping bag with star nozzle fitted.

Gently take one biscuit and turn upside down. Spread 1 tsp of jam onto the biscuit, then pipe on the buttercream in a swirl. Place a second biscuit on top (swirl-side-up) and very gently press down to create your sandwich. Repeat until all the biscuits are filled.