Heat the oven to 180C/160C fan/gas 4. Tip all the sugars and the butter into a bowl and beat using an electric whisk (or do this in a stand mixer) until lighter in colour, about 3-4 mins. Add the egg and vanilla, then mix again until fully combined. Scrape down the sides of the bowl and stir in the flour, bicarbonate of soda and 3/4 tsp salt. Fold everything together until mostly combined, then add the lightly crushed Smarties and freeze-dried raspberry pieces, if using. Mix until these are evenly dispersed throughout the dough.

Shape the dough into 14-16 equal balls (about 60g each), then gently press 3-4 whole Smarties and some sprinkles (if using) into each. Transfer to a baking sheet lined with baking parchment and chill for at least 1 hr, or overnight for best results. (The cookies can be baked straightaway, but they will spread more if they haven't been chilled.)

Bake for 12-15 mins until golden and still slightly soft in the middle. Cool completely before serving. Will keep in an airtight container for up to five days, or frozen for up to three months. Defrost completely before serving.