• Place the chilled butter and icing sugar into the Thermomix bowl. Mix for 40 sec/speed 4 until softened. Scrape down the sides of the mixing bowl with a spatula.
• Add the flour, cornflour and vanilla extract. Mix for 15 sec/speed 4, scrape down the sides of the bowl, then knead for 1 min/dough function. The dough will have come together.
• Roll about 1 tablespoon of dough into balls. Arrange them on a large baking tray lined with baking paper, leaving 3cm between each one. You may need two trays, depending on the size of your biscuits. Gently press each ball with a floured fork to flatten slightly, then refrigerate for 15 minutes so they hold their shape while baking.
• Preheat the oven to 170°C/340°F (150°C fan).
• Bake the melting moments for 13–15 minutes, until they are firm around the edges but still pale and slightly soft in the centre (if using two trays, bake one at a time). They’ll firm as they cool. Leave them on the tray for 10 minutes, then transfer to a wire rack to cool completely. They’ll be very delicate when warm but will set once cooled. Make sure they are completely cool before icing.

For the Lemon Buttercream Filling

• Place the icing sugar and softened butter into the Thermomix bowl. Mix for 15 sec/speed 6. Scrape down the sides of the bowl with a spatula.
• Insert the butterfly whisk. Add the lemon juice and mix for 1 min/speed 3 until creamy. If the mixture is too soft to spread, place it in the fridge to firm up. Any leftover buttercream can be stored in the fridge.

To Assemble

• Spread a generous amount of the lemon buttercream onto the underside of half the biscuits, then sandwich with the remaining biscuits. Gently press together until the filling reaches the edges. If the buttercream is too soft to spread, chill it briefly in the fridge to firm up.