Crush shortbread (not too finely) and mix with cocoa. Melt butter, syrup and sugar in a pan, then add biscuit/cocoa mix gradually, stirring well to mix. Press into buttered 8x8" square dish and leave to cool.
Melt chocolate, milk and butter gently together, stirring. Spread icing over cookies. Decorate with sprinkles/jimmies, silver dragées, mini-M&Ms, etc, if desired. Cut into bars.