Preheat oven to 400 degrees. Line a rimmed 15 by 10-inch jelly roll sheet* (this size HERE - affiliate link) with aluminum foil. Spray foil with non-stick cooking spray.
Align crackers in a single layer on prepared baking sheet, set aside.
Add butter and brown sugar to a medium saucepan (2.5 - 3 quart). Heat mixture over medium heat. Cook stirring constantly until mixture reaches a full boil. Once it boils stop stirring and let boil 3 minutes.
Remove from heat and stir in vanilla (stand back it will sputter!).
Immediately (and carefully) hot mixture evenly over crackers. Spread evenly as needed.
Bake in preheated oven 5 minutes. Carefully remove from oven (this is boiling hot sugar)**.
Right away sprinkle chocolate chips over mixture. Let rest 5 minutes to allow chocolate to melt***.
Spread chocolate into an even layer using an offset spatula, sprinkle evenly with nuts or sprinkles if using.
Transfer to fridge and let chocolate harden, about 30 minutes, or freeze for 15 minutes.
Remove from fridge, lift cracker from foil and break into pieces. Store in an airtight container (I like to refrigerate though it's not a must).
Finish with white chocolate chips instead of milk or semi-sweet chocolate chips. Then you can also dust with cinnamon or pumpkin pie spice.
Use chopped walnuts or pistachios or sliced almonds instead of pecans.
Decorate with festive mini M&M's.
Crush up Oreos and sprinkle over chocolate.
After chocolate layer has set drizzle with melted white chocolate for a pretty finishing touch.
Use graham crackers in place of saltine crackers.
Finish with crushed toffee bits for even more toffee flavor.