Grease an 8x8 inch square pan with butter. Grease a heavy-bottomed saucepan with butter. Grate the chocolate into it and add the sugar, milk and corn syrup. Stir over low heat until chocolate melts and sugar dissolves. Increase the heat to medium-high and boil the mixture until it reaches soft ball stage (240F.). Stir just enough to prevent scorching., remove from the heat and add the butter. Cool to lukewarm (110F.) without stirring. Add the vanilla. Beat until the candy loses its gloss and becomes thick enough to hold its shape. This will take 10 minutes if you use a heavy electric mixer and longer by hand. Immediately pat into the prepared pan. Cool and cut into squares. Yields 36 pieces.