Grease a 23cm (9 in) square baking tin and line with greaseproof paper.
In a microwave-safe bowl, combine 500g chocolate and sweetened condensed milk. Heat in microwave until chocolate melts, stirring occasionally. Be careful not to let it seize. Stir in vanilla and salt. Spread into tin and cool to room temperature.
In a microwave-safe bowl, combine cream, liqueur and 350g chocolate. Heat in microwave until the chocolate melts; stir until smooth. Cool to lukewarm then pour over the fudge layer. Refrigerate until both layers are completely set, about 1 hour. Cut into 40 squares.