Preheat the oven to 180°C/350°F (160°C fan). Line 2 baking trays with baking paper.
Add the butter to the Thermomix bowl. Mix for 12 sec/speed 5 until softened and spreadable. Scrape down the sides of the mixing bowl with a spatula.
Add both sugars to the mixing bowl. Mix for 20 sec/speed 4. Scrape the bowl thoroughly, including under the blades. Mix again for 15 sec/speed 4, until smooth and creamy. Note: Just before the end, the blades may spin freely, this is normal.
Add the egg and vanilla extract. Mix for 15 sec/speed 3. Scrape down the sides of the bowl and mix again for 15 sec/speed 3.
Add the flour, bicarbonate of soda and salt. Mix for 10 sec/speed 4, scrape down the bowl and mix again for 3 sec/speed 4.
Add the white chocolate chips and macadamia nuts to the Thermomix bowl, then stir them in with a spatula (so they don't get crushed by the blades). Tip: if stirring in the Thermomix bowl feels fiddly with the blades in the way, transfer the dough to a regular mixing bowl and stir them in there instead.
Roll dough into walnut-sized balls, place 5 cm apart. If the dough feels too sticky to roll, pop it in the fridge for 10-15 minutes to firm up. Carefully press some extra white choc chips on top, there's no need to press the cookie dough down. Bake one tray at a time for 12-14 minutes, until golden around the edges but still soft in the centre. Keep the second tray in the fridge while the first bakes.
Leave cookies on trays for 5-10 minutes before transferring to a wire rack. They will firm up as they cool.
Notes
Storage: Store the cookies in an airtight container at room temperature for up to 5 days. Freeze the baked cookies for up to 2 months, or freeze the unbaked cookie dough balls and bake straight from frozen, adding a minute or two to the baking time.