Line a 9x13 baking pan with foil, extending foil over the edges to contain the hot toffee mixture.
In a heavy medium saucepan (2 quart), melt butter over medium heat.
1 cup Unsalted Butter
When melted, add sugar, salt, corn syrup, and water. Cook and stir using a wooden spoon or rubber spatula over medium-high heat to boiling.
1 cup White Sugar, 1 pinch Salt, 1 Tablespoon Light Corn Syrup, 3 Tablespoons Water
As soon as it starts boiling, STOP STIRRING.
Clip the candy thermometer to the sides of the pan. Cook until the temperature reaches 290 degrees. This will take 10-15 minutes. Watch carefully after 280 degrees to prevent scorching.
Remove the saucepan from the heat. Remove thermometer.
Pour sugar mixture into the prepared 9x13 pan.
Let stand 5 minutes or until firm; sprinkle with chopped chocolate pieces in an even layer.
1 cup Dark Chocolate
Let stand for 1-2 minutes to allow chocolate to melt. Using an offset spatula, spread the chocolate evenly. Sprinkle a single layer of pecans and flake sea salt over melted chocolate.
½ cup Chopped Pecans, 1 teaspoon Flake Sea Salt
Chill until firm.
After the toffee is cool, lift the candy out of the pan and break into pieces.