Preheat oven to 180°C. Line oven trays with baking paper.
Beat butter, sugar, egg and extract in a small bowl with an electric mixer until combined; stir in sifted flour, in two batches.
Divide dough in half; stir sifted cocoa and milk into one portion. Cover; refrigerate dough for 30 minutes.
Roll dough portions, separately, between sheets of baking paper until 5mm thick. Cut 15 x 6cm rounds from each; place 2.5cm apart on trays.
Press white choc Bits onto chocolate rounds to make crosses; press dark choc Bits onto vanilla rounds to make noughts. Bake for 15 minutes or until a biscuit can be pushed gently without breaking; cool on trays.