Preheat the oven to 190°C (170fan/ 375F/ Gas 5) and line 2 large baking trays with baking paper.
In a large bowl cream together the softened butter and sugars until pale and fluffy.
150 g Unsalted Butter, 80 g Light Brown Sugar, 80 g Caster Sugar
Beat in the egg and vanilla extract.
2 teaspoon Vanilla Extract, 1 Egg
Mix in the flour, bicarbonate of soda and sea salt.
225 g Plain Flour, ½ teaspoon Bicarbonate of Soda, ¼ teaspoon Salt
Gently fold in three-quarters of the smarties
200 g Smarties
Use a tablespoon to take small scoops of the mixture, roll them into balls with your hands, and place them on lined baking trays spaced well apart. This mixture should make about 24 cookies. Press the rest of the smarties on top.
Bake in the oven for 12 minutes until lightly golden.