Preheat oven to 190C..
Sieve the flour, cocoa powder, salt and baking soda a few times and keep aside.
Beat the sugar and butter till creamy.
Add the milk and vanilla essence and mix well.
Add the flour mixture and mix till well blended.
Take a spoonful of mixture and roll into a ball.
Make a depression in the centre and place a chunk of chocolate.
Cover and roll into a ball and flatten it.
Place on a lined cookie sheet.
Bake in the oven for about 12 - 15 minutes.
Leave it to cool.