Heat oven to 350F and grease a 13X9 inch baking pan. In a bowl, combine the butter, sugar, egg yolk and vanilla. Blend in the flour and salt. Press mixture onto the bottom of the prepared dish. Bake for 25 to 30 minutes until lightly browned. Remove from the oven and sprinkle on the chocolate. Let stand until the chocolate is soft and spread evenly over the crust. Sprinkle pecans over top and let cool.