In a large bowl, stir together the icing sugar, cocoa, flour, bicarb soda and salt until well blended. Cut or rub in the butter until mixture resembles coarse breadcrumbs. Add eggs and vanilla; mix until a stiff dough forms. It may take a minute to come together.
On a lightly floured surface, roll out dough to 1/2cm thickness and cut into desired shapes using biscuit cutters. If the dough is too sticky, chill for a little bit. Place biscuits 5cm apart onto an ungreased baking tray.
Bake for 8 to 10 minutes in the preheated oven, or until the surface appears dry. Allow biscuits to cool for a couple minutes on the baking tray before removing to wire racks to cool completely.
If adding the melted chocolate, wait till biscuits are completely cool, melt the chocolate over a double boiler or in the microwave. Stir frequently until smooth. Dip cookies or drizzle with the chocolate and place on greaseproof paper to set.
I noticed that the oven temp is not listed, and when to add the bicarb soda. I baked mine at 190oC and added the bicarb with other dry ingreds. Still was too sticky to roll out, so I rolled into balls and flattened with a fork. Tasted nice! - 30 Nov 2010