Line an 8-by-8-inch baking pan with aluminum foil and coat it with nonstick cooking spray.
Combine chocolate, sugar, milk, cream, corn syrup, and salt in a saucepan. Stir over medium-low heat with a wooden spoon until the chocolate melts and the mixture begins to boil.
Continue cooking without stirring until it reaches 236°F, soft ball stage.
Remove from heat and add the butter. Do not stir. Let mixture cook to about 110°F.
Stir in the vanilla and nuts and beat vigorously with a wooden spoon or in a stand mixer until the mixture becomes thick and loses its shine.
Pour into the baking pan. Let set for 1 to 2 hours until firm. Cut into squares.
Store for up to 2 weeks in an airtight container.