Heat the oven to 350°F (175°C). Line a rimmed 10 x 15-inch baking sheet with parchment paper or foil, then lightly grease the lining.

Arrange 40 saltine crackers in one tightly fitted layer on the prepared baking sheet. Let the crackers touch or slightly overlap so the caramel stays over the cracker base.

Melt the butter and packed brown sugar in a saucepan over medium heat. Once fully melted, bring the mixture to a steady boil and cook for about 3 minutes, stirring frequently, until smooth and slightly thickened; stir in vanilla if using.

Pour the hot caramel evenly over the crackers and spread it carefully with a spatula, covering as much of the surface as possible.

Bake for 5 to 7 minutes, until the caramel is bubbling across the surface. Watch closely near the end of baking.

Remove the pan from the oven and immediately sprinkle the chocolate chips over the hot caramel. Let them stand for 2 to 3 minutes, then spread the softened chocolate into an even layer.

Sprinkle with chopped pecans or walnuts and flaky sea salt, if desired. Cool at room temperature or refrigerate until firm, then break or cut into pieces.