Using an electric mixer with a paddle, cream butter and sugar on medium speed until light and fluffy. Add egg yolk and vanilla essence.
In a separate mixing bowl, mix flour, corn flour and cocoa powder. Add dry ingredients to the butter mixture to form a soft dough.
Fill the dough into a tube semperit maker from the wider end. Using the wooden pin, compress the dough to expel all air and ensure dough is compact right up to the end of the tube.
Hold the tube vertically with the open end facing up. Push out a bit of dough. Run an upright, blunt knife across the open end to scrape the dough off - this helps the dough below to form ridges as it is pushed out of the tube. Still holding the tube vertically, slowly push out dough to a height of about 1cm by pushing the tube down against the wooden pin. Stop and slice dough by running a knife through it.
Arrange ridged cookies on baking tray. Continue until all dough is used up.
Preheat oven to 180°C. Bake cookies for about 20 minutes or until lightly browned.