Step 1 Place the sugars and butter in the bowl of an electric mixer (or use an electric hand beater), and beat until pale and fluffy. Use a whisk to beat together the eggs and vanilla in a jug, then gradually transfer to the butter mixture, a little at a time, beating between each addition (this prevents the mixture from curdling).
Step 2 Sift the flour, baking powder, soda and a pinch of salt over the mixture, then use a metal spoon to carefully fold together until there are no lumps of flour in the mixture. Gradually add the milk, stirring until you have a soft dough (don’t add too much milk or it will be too sticky). Use your hands to roll the mixture into 2 (about 30cm long) logs. Wrap each in plastic wrap and freeze for 1 hour.
Step 3 Preheat the oven to 200°C. Lightly grease 2 large non-stick baking sheets or trays. Use a serrated knife to cut 5mm-thick discs from the dough and use a rolling pin or the flat of your hand to slightly flatten each disc. Transfer to the baking sheets, leaving space between them as they will spread on cooking. Press 3 or 4 Smarties into the top of each biscuit and bake for 8-9 minutes or until the biscuits are golden brown. (They will still be a little soft at this stage so transfer to a wire rack to cool completely.)