Beat butter, vanilla extract and sugar in a small bowl with an electric mixer until smooth.
Stir in combined sifted flour, cocoa, bicarbonate of soda, and chocolate. Mix with a spoon until combined.
Roll level tablespoons of mixture into balls. Place balls, about 4cm apart, on oven trays lined with baking paper. Flatten each one with a fork to form a criss-cross pattern.
Cook in a moderately slow oven (160°C) for about 10 minutes, or until lightly browned. Allow biscuits to cool on trays.