Preheat oven to 400º. Line a 18x13 baking sheet with aluminum foil and spray with non stick cooking spray. Lay 40 Saltine crackers out on baking sheet. They should fill the baking sheet perfectly.
In a medium sized pot, heat butter and dark brown sugar over medium heat until melted. Then increase heat to high until mixture starts to simmer. Let mixture boil for exactly 3 minutes. At three minute mark, turn off heat and quickly spoon toffee mixture over each cracker.
Carefully place sheet pan in oven and let cook for another 5 minutes. Mixture should be golden brown and bubbly when removed. Remove from oven, let cool for 1 minute then evenly top with one bag of chocolate chips. Let chocolate chips sit for 5 minutes, or until melted. Spread melted chocolate over top of crackers. Sprinkle with chopped pecans. Refrigerate until chilled and break into pieces.