Preheat oven to 400 degrees F. Line a 10x15 jelly roll pan with foil. Lightly grease the foil with cooking spray. Arrange the crackers in an even layer side-by-side on the baking sheet, breaking up crackers to fit if necessary. Set aside.

In a large saucepan, combine the butter, brown sugar and vanilla over medium heat. Cook and stir until melted, then bring to a boil and boil for exactly 3 minutes, stirring constantly. Remove from heat and immediately pour the hot toffee mixture evenly over the crackers. Use an offset spatula to carefully spread the toffee around, but if the toffee doesn't cover all the crackers, it's okay - as it bakes it will spread some.

Immediately place the toffee into the oven and bake for exactly 7 minutes. Evenly sprinkle the bag of chocolate chips over the toffee mixture and return to the oven for 1-2 minutes or until chocolate is glossy. Remove from oven and spread the chocolate chips around to create a thin chocolate layer using an offset spatula. Sprinkle with the candies and sprinkles, if using.

Allow the toffee to come almost to room temperature before placing in the fridge to set for about 10-15 minutes before breaking into squares.