Mixing the Dough: Combine the plain flour, cold butter (cut into cubes), sugar, egg, and salt. Mix until the dough just comes together. If you are making the dark version, add the 20g of cocoa powder at this stage.
Adding the Fillings: Gently fold in your 120g of finely chopped mix-ins.
Shaping: Divide the dough into two equal pieces and roll each one into a firm log shape.
Chilling: Wrap the logs tightly in cling film. Place them in the fridge to set for a minimum of 4 hours, or pop them in the freezer for 1 hour if you are short on time.
Slicing: Once the dough is completely firm, use a sharp knife to cut the logs into even, 0.5 cm thick rounds.
Baking: Place the cookies on a baking sheet lined with parchment paper. Bake in a preheated oven at 160 °C (fan/convection setting) for about 14 -17 minutes.
Cooling: Let them cool on a wire rack to crisp up perfectly.