Lightly grease a baking sheet. Arrange pecans in an approximate 10 x 12 inch rectangle.
Melt butter on low in a heavy saucepan. Off heat, stir in sugar until completely dissolved. Return pan to burner on medium heat and cook, stirring occasionally until a candy thermometer reaches 300 degrees, hard crack stage. Off heat, add vanilla, stirring it in quickly. Immediately pour toffee over pecans, starting in the middle and working toward the edges.
After 3 minutes, sprinkle chocolate over toffee. When it melts, spread it out evenly with a small spatula. Let cool for 3 hours then break into pieces. Keeps in an airtight container for weeks.
Yields 1 pound