In a bowl, beat the butter and powdered sugar until smooth, and light and creamy. Add the vanilla and salt, and continue beating. Slowly add the cocoa powder and flour (sift if it is lumpy) and mix together well. Stir in the choc chips so they are evenly distributed.
Lay the dough on baking paper or cling wrap, and roll into a log. The thickness of the log depends on how big you want the shortbread to be - I made roll about 6cm thick. Place this in the fridge for 30 minutes, or the freezer for 15.
Preheat the oven to 180C.
Line two trays with baking paper, then unroll the cold cookie dough log, and cut into 1 to 1.5 cm thick pieces with a bread knife. Place the shortbread on the tray and bake for 15 - 20 minutes. If you made large ones, leave them in the oven longer. Allow to cool on a wire rack.