preheat oven to 180°C/350°F (not fan-forced)
sift flour and bi carb soda and set aside
in the bowl of a stand mixer, cream butter, sugar and salt on medium speed, until light and fluffy. You will need to stop the mixer a couple of times and scrape the bowl.
turn mixer to low and add the egg, egg yolk and vanilla. Mix until combined.
turn the mixer off, then add the sifted flour and bicarb
mix on low, until almost combined
Add 150g of Smarties and mix on low until just combined
Using a large ice cream scoop, scoop out dough and place onto lined baking trays, leaving space for the cookies to spread slightly (I put 9 on a tray)
Optional: press 4 or 5 Smarties into the top of the cookie dough balls (to ensure the Smarties are visible after baking)
Bake cookies for 8 minutes, until lightly golden but still soft
Remove from the oven and quickly and carefully press each cookie to flatten slightly, then reshape if necessary (you can do this with a spatula, or twil them inside a large round cookie cutter).
Slide baking paper off the tray and allow cookies to set on a cool surface
Cookies are best eaten within a couple of days and can be frozen for a couple of months