In a large heavy saucepan, cook the sugars, milk, corn syrup, and salt until sugars dissolve and mixture boils. Continue boiling, stirring constantly until mixture reaches 236 degrees on a candy thermometer (soft ball stage).
Remove from heat plop in the peanut butter, but do not stir in.Cool without moving pan til mixture reaches 115 degrees, this could take as long as an hour.
Add vanilla, and beat until mixture thickens and turns lighter in color, should just take a few minutes.
While mixture still flows, and hasn't lost it's gloss, pour into the prepared pan.
Allow to cool completely, then loosen edges with a sharp knife, turn out on a cutting board and cut into 1 inch squares.