Preheat oven to 180c. Put unsalted butter, sifted icing sugar and salt in the bowl of an electric mixer and beat on high for 2 minutes or until light and creamy.
Add vanilla extract and yolk and continue beating for 1 minute. Turn off mixer and gently fold in flour with a wooden spoon.
Put 1/4 of the dough in a separate bowl and fold in cocoa. Gently mix cocoa dough into vanilla dough to create a marbled effect.
On a lightly floured bench, roll out dough to 5 mm thick and cut out shapes. Line oven tray with baking paper. Lay your dough shapes on tray, and then put in freezer for 3 minutes.
Bake for 8-9 minutes until dough is golden and biscuit is slightly firm to touch. Cool biscuits on wire rack, then serve.