Line a jelly roll pan (10 x 15 inches) with nonstick foil or a silicone baking mat. Preheat the oven to 350°F.
Arrange the saltine crackers in a single layer on the pan.
In a 3-quart heavy-bottomed saucepan, melt the butter and brown sugar over medium-high heat, stirring frequently. Make sure to keep stirring to prevent the sugar from burning!
Bring the mixture to a boil and let it boil for 3 minutes, stirring almost continuously. If using a candy thermometer, aim for 270°F-290°F.
Remove the saucepan from the heat and stir in the vanilla extract and salt.
Pour the caramel mixture over the saltines, spreading it evenly.
Bake for 5 minutes until the caramel is bubbling vigorously. If using a candy thermometer, aim for 300°F-310°F.
Sprinkle the chocolate chips over the hot caramel, cover the pan with foil, and let it sit for 5 minutes to allow the chocolate to melt.
Remove the foil and use an offset spatula to spread the melted chocolate evenly over the top.
Add any desired toppings, like chopped nuts or sprinkles, and let the toffee cool at room temperature until set, or chill in the fridge for at least 2 hours.
Peel away the foil from the bottom of the toffee and either crack it by hand or cut it into pieces with a knife.
Store the toffee in an airtight container in the refrigerator for up to 1 week.
Notes
You can customize this toffee by adding chopped nuts, sea salt, or sprinkles on top of the chocolate before it sets.