Preheat the oven to 180C/350F (160C/320F fan-assisted).
In a large bowl, beat together the butter, golden syrup and sugar until fluffy.
Beat in the egg and the vanilla until well combined.
Add the flour and the baking powder and mix well – you should now have a sticky, light dough.
Fold in the white chocolate chunks and all but a handful of the M&Ms.
Use a tablespoon to scoop the dough, flattening off against the side of the bowl, then roll the dough into a ball with your hands. This is the best way to get 16 equally sized cookies.
Place the balls of dough on baking sheets lined with greaseproof paper, being sure to place each cookie at least an inch apart so that they don’t stick together when they spread.
Pop a 3 extra M&Ms on top of each cookie.
Bake for 8-10 minutes until golden at the edges. Get them out of the oven before they’re fully browned so that they stay chewy when cool.
Allow to cool on the tray until firm enough to transfer to a wire rack to cool completely.