Prepare your candy making area first. Arrange your cookware, utensils and measure out all ingredients ahead of time so you can work quickly. Spray interior surface of 9” x 13”x 1” (23cm x 33cm x 2.5cm) baking pan with cooking spray. Fill a small bowl with water for use with a pastry brush.
Roughly chop 2 cups of the whole pecans and then finely chop the remaining 1 cup of pecans. Place the 2 cups of roughly chopped pecans in the prepared baking pan, distributing the pecans evenly across the pan. Set aside.
Place the butter and sugar in the saucepan or Dutch oven and heat over a medium low heat. The butter and sugar will begin to melt together. Gently stir the mixture with a heat proof rubber spatula. Place a candy thermometer in cooking pan. Watch that the mixture does not build up against the side of the pan. If so, use the pastry brush dipped lightly in water to brush down the interior of the pan.
Once the mixture reaches the soft ball stage (234 to 241°F (112 to 116 °C)) stir the mixture consistently and smoothly. Avoid touching or splashing the mixture or stirring too quickly to avoid burns on your skin. Continue to watch for sugar building up on the sides of the pan which should be brushed down with the moistened pastry brush. The mixture should turn golden brown. The cooking time is approximately 10 minutes.
Once the mixture reaches the hard crack stage (295 to 309 °F (146 to 154 °C)) remove the pan from the heat and carefully pour in the vanilla extract. Mix gently but thoroughly.
Carefully pour the toffee mixture into the prepared pan filled with pecans. You can tilt the filled pan to fill every corner with toffee but be careful as molten sugar can burn your skin.
After 1-2 minutes the toffee should harden but still be hot to the touch. Evenly sprinkle over the chocolate chips and allow to rest for 1-2 minutes. The chocolate chips will begin to melt. Using a pastry spatula, lightly smooth over the chocolate chips so you spread the melting chocolate evenly over the toffee. Allow to rest for another 2 minutes. Sprinkle over the finely ground pecans evenly and allow the toffee to rest until completely cool for several hours or overnight.
Break into pieces and store in an airtight container.