Melt butter, milk chocolate and sugar in a bowl over a pan of simmering water. Leave to cool
Stir in vanilla & salt, followed by digestives and 220g maltesers.
Pour into greased baking tin and chill for at least 3 hours or until set. Once set melt white chocolate and pour an even layer over the top.
Crush remaining maltesers and sprinkle over the top. Chill, then cut into bars as soon as white chocolate is set.