In a large heavy saucepan, combine the butter, sugar and milk. Cook and stir over low heat until sugar is dissolved. Bring to a boil; boil and stir for 4 minutes. Add dates; boil and stir for 1 minute.
Remove from the heat; stir in candy coating and marshmallows until melted. Beat until smooth. Add pecans and vanilla; beat with a wooden spoon until glossy.
Pour into a buttered 13-in. x 9-in. pan. Let stand at room temperature overnight. Cut into squares.