Beat the butter and sugars for about 2 minutes until creamy and slightly lighter.
Beat in the egg, yolk and vanilla until combined. Add malt extract and mix until combined.
Whisk together the flour, bicarbonate of soda, baking powder and salt.
Add to the butter mixture and mix just until almost combined. Fold through the chopped Maltesers.
Portion into 60–65g balls and refrigerate for 30–45 minutes.
Heat oven to 175°C fan / 190°C conventional and line baking trays. Space the cookies well apart and bake for 10–12 minutes.
Finish with a little flaky salt and leave on the tray for 8–10 minutes before transferring to a rack.