Preheat the oven to 170°C (150 fan) | 325°F | gas 3 and line two baking trays with silicone paper or non-stick baking mats.
Mix together the sugars and butter until smooth then beat in the egg, egg yolk and vanilla extract. Sift in the flour and salt and add half of the Smarties. Stir together until just combined.
Use an ice cream scoop to portion the dough onto the baking sheets, leaving plenty of room for each cookie to spread. Press a few extra Smarties into the top of each cookie.
Bake for 15 - 17 minutes or until the edges are golden and the centres still a little soft. Leave the cookies on the baking sheet for a few minutes to firm up before transferring to a wire rack to cool.
Repeat as necessary until the dough has all been used.