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 setting. 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 Buttercream Filling: Place the icing sugar and softened butter into the Thermomix bowl and mix for 15 sec/speed 6. Scrape down the sides of the mixing bowl with a spatula.
Insert the butterfly whisk. Add 1 teaspoon of boiling water and ¼ teaspoon of vanilla extract to the icing mixture, then mix for 30 sec/speed 3. Scrape down the sides of the bowl and mix again for 20 sec/speed 3.
To Assemble: Spoon generous amounts of buttercream onto the underside of half the biscuits and sandwich with the remaining biscuits. Press down gently so a little buttercream seeps over the edges, then dip in the sprinkles. If the buttercream feels too soft to spread, place it in the fridge for a while to firm up.
Notes
Recipe Tips
Chill the unbaked biscuits before baking so they hold their shape.
Bake until pale and just set - they'll firm as they cool.
Ensure they're completely cool before filling.
Storage: Store the melting moments in an airtight container at room temperature for up to 5 days, or in the fridge for 1 week. Freeze unfilled biscuits for up to 2 months, then thaw and fill before serving.