Pre-heat your oven to 180°c (160°c for fan-assisted ovens or Gas Mark 4) and grease a couple of baking sheets with a little butter/margarine. Set aside.
In a large bowl, cream the butter/margarine, sugar and vanilla extract, until light and fluffy.
Gradually add the flour & drinking chocolate, a little at a time, until the mixture forms a paste.
Divide into small balls (about a teaspoon-tablespoon per biscuit) and place onto your pre-greased baking sheet with a bit of space between them. Flatten each ball lightly with a fork.
Bake in your pre-heated oven for 12-15 minutes until the appear dry on top. Leave to cool a little on the baking sheets, before transferring to a wire rack to cool completely.
Mix the butter/margarine and icing sugar to form the butter cream. Once the biscuits are completely cool, pipe or spread a little butter cream onto half the biscuits and sandwich together with the other halves.