Preheat your oven to 180°C/160°C Fan/Gas 4 and line a baking sheet with parchment paper. If you don't have any paper you can slightly grease it.
In a mixing bowl, cream together the softened unsalted butter and the granulated sweetener until well combined.
Add the egg, yoghurt and vanilla extract to the butter sweetener mixture and mix until fully combined.
In a separate bowl, whisk together the whole wheat flour, baking powder, and a pinch of salt.
Gradually add the dry ingredients to the wet mixture, stirring until a cookie dough forms. If the dough appears too sticky, you can add a little more flour (10-20g) until it reaches a workable consistency.
Gently fold in the broken Smarties.
Drop spoonfuls of cookie dough onto the prepared baking sheet, leaving some space between each cookie.
Use a fork or the back of a spoon to flatten each cookie slightly, as they won't spread much during baking.
Bake in the preheated oven for 10-12 minutes, or until the edges of the cookies are lightly golden brown.
Remove the cookies from the oven and let them cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.