Preheat an oven to 350℉. Line a large cookie sheet with parchment paper, then evenly space the saltine crackers in rows on top. Set aside.
Place the butter and brown sugar into a saucepan over medium heat. Let the mixture come to a boil for two to three minutes (around 270-290℉ on a candy thermometer, but it doesn't need to be exact), then carefully stir in the vanilla and salt.
Pour the toffee mixture all over the saltines, quickly spreading it until evenly coated, then place the tray into the oven. Bake the crackers for 5-10 minutes, or until the toffee mixture is bubbling all over. Remove the pan from the oven.
Place the chocolate chips into a heat-safe bowl and microwave in 30-second increments until fully melted. Pour the chocolate onto the saltines and use an offset spatula to evenly spread it over the crackers.
Alternatively, you can scatter the chocolate chips over the toffee layer immediately out of the oven, then let it rest for 5 minutes and spread the chocolate until smooth (if not fully melted, place the pan back into the oven for a few minutes).
At this point, add your toppings of choice and let everything cool completely before breaking into pieces. Store in an airtight container at room temperature for up to one week.