Heat your oven to 160°C fan, or 180°C conventional. Line two baking trays with baking paper.
Place the chilled butter in the Thermomix and blend for 12 sec/speed 5 until smooth and workable. Scrape down the sides of the bowl with a spatula.
Add the brown and white sugar, then mix for 20 sec/speed 4. Scrape the bowl thoroughly, then mix again for 15 sec/speed 4 until smooth.
Add the egg and vanilla, then mix for 15 sec/speed 3. Scrape down the bowl, then mix again for 15 sec/speed 3.
Add the flour, bicarbonate of soda and salt. Mix for 10 sec/speed 4, scrape the bowl well, then mix again for 3 sec/speed 4. Lift the mixture around the blades to make sure no dry flour is hiding underneath.
Fold the Smarties through by hand using a spatula so they stay whole. Tip: For easier mixing, transfer the dough to a regular mixing bowl where you can stir freely without the blades. I also find it easier to scoop the dough this way.
Shape the dough into walnut-sized balls and arrange them on the trays, leaving about 5 cm between each one. Press 2-3 extra Smarties on top. If the dough feels soft, place it in the fridge for 20 minutes to chill.
Bake one tray at a time for 12–14 minutes, until the edges are lightly golden and the centres remain soft. Keep the second tray in the fridge so the dough stays firm.
Allow the biscuits to cool on the trays for 5 minutes, then carefully lift them with a spatula onto a wire rack. They will firm up as they cool.