You will need a thermometer that registers high temperatures for this recipe.
Make foil sling for 13 by 9-inch baking pan by folding 2 long sheets of aluminum foil; first sheet should be 13 inches wide and second sheet should be 9 inches wide. Lay sheets of foil in pan perpendicular to each other, with extra foil hanging over edges of pan. Push foil into corners and up sides of pan, smoothing foil flush to pan. Spray foil with vegetable oil spray.
Heat butter and water in medium saucepan over medium-high heat until butter is melted. Pour sugar and salt into center of saucepan, taking care not to let sugar touch sides of saucepan. Bring mixture to boil and cook, without stirring, until sugar is completely dissolved and syrup is faint golden color and registers 300 degrees, about 10 minutes.
Reduce heat to medium-low and continue to cook, gently swirling saucepan, until toffee is amber-colored and registers 325 degrees, 1 to 3 minutes longer. Off heat, stir in 1/2 cup pecans until incorporated and thoroughly coated.
Pour toffee into prepared pan and smooth into even layer with spatula. Refrigerate, uncovered, until toffee has hardened, about 15 minutes.
Microwave 4 ounces chocolate in bowl at 50 percent power, stirring occasionally, until melted, about 2 minutes. Pour chocolate over hardened toffee and smooth with spatula, making sure to cover toffee layer evenly and completely. Sprinkle with ½ cup pecans and press lightly to adhere. Refrigerate, uncovered, until chocolate has hardened, about 15 minutes.
Line rimmed baking sheet with parchment paper. Using foil sling, invert toffee onto prepared sheet. Discard foil.
Break bark into rough squares and serve. (Bark can be stored at room temperature for up to 2 weeks.)