Preheat oven to 190 degrees celsius.
Line 2 large baking trays with baking paper and set aside.
Place butter, brown sugar and caster sugar into the TM bowl and mix for 1 minute, Speed 6.
Reduce to Speed 3, and while the blades are turning, add the salt, vanilla extract, and eggs through the MC hole.
Add the plain flour and bi-carb soda and mix for 40 seconds, Speed 4.
Place heaped teaspoon sized balls of the mixture onto the prepared baking trays (allow plenty of room for spreading).
Arrange several Smarties on top of each cookie.
Bake for 8-10 minutes or until the edges have just started to appear golden (don't overcook). *If you want to add extra Smarties, take the cookies out halfway through cooking and add a few extras - by this stage the cookies will have spread and you'll have lots more room for extra chocolatey goodness!).
Remove from the oven and leave on the baking trays for 10 minutes.
Carefully use a flat spatula or scraper to move the cookies to a wire rack to cool completely.
Store cookies in an airtight container at room temperature for up to 5 days.