Line a 9-by-13-inch pan with foil; spray it with vegetable oil. In a heavy saucepan, melt the butter. Add the sugar, corn syrup and cream and bring to a boil, stirring until the sugar dissolves. Add the vanilla seeds. Cook over moderately low heat, stirring, until a golden caramel forms and the temperature reaches 245° on a candy thermometer, 1 hour. Stir in 1 tablespoon of salt and scrape the caramel into the prepared pan. Let cool and set completely overnight.
Lightly oil a sheet of parchment paper and line 2 baking sheets with wax paper. Invert the caramel onto the parchment and peel off the foil. Using a sharp knife, cut the caramel into 1-inch-wide strips and then into 1-inch squares. Dip the squares into the chocolate, tap off the excess and transfer to the wax paper on the baking sheets. Sprinkle lightly with sea salt and refrigerate for 10 minutes. Alternatively, wrap the plain caramel squares in wax paper and tie with thread.