Preheat oven to 400°F (200°C). Line a 10×15-inch rimmed jelly roll pan with aluminum foil and lightly coat with nonstick cooking spray.
Arrange the saltine crackers in a single layer across the prepared pan.
In a saucepan over medium heat, melt butter and brown sugar together.
Bring to a steady boil and cook for approximately 3 minutes, stirring constantly.
Remove from heat and carefully stir in vanilla extract.
Immediately pour the hot toffee evenly over the crackers and spread with a heat-resistant spatula.
Bake for approximately 5 minutes, until the toffee is bubbly.
Remove from the oven and sprinkle chocolate chips evenly over the hot toffee.
Let sit for approximately 5 minutes, then spread the softened chocolate into a smooth layer.
Sprinkle chopped pecans evenly over the chocolate.
Cool until no longer hot, then refrigerate for approximately 1 hour or until completely firm.
Break into pieces or cut into squares before serving.