This is the most wonderful mixer, we believe in having ...
Melt chocolate, condensed milk together in a heavy saucepan over low heat (or microwave in a large bowl) - stir until smooth. Remove from heat and stir in pecans and vanilla. Spread evenly into a tin foil lined 20 or 23 cm (approximately) square pan. Chill for at least 2 hours until really firm. Cut with a warm knife.