Preheat oven to 350°F. Line a 9 x 13-inch baking pan with aluminum foil, leaving some overhang for easy lifting, and spray the foil well with cooking spray.

Arrange 30 to 40 saltine crackers in a single layer to cover the bottom of the prepared pan.

In a medium saucepan, combine 1 cup unsalted butter and 1 cup packed brown sugar. Cook over medium heat, stirring constantly, until the mixture comes to a boil.

Once boiling, reduce the heat so the mixture maintains a gentle simmer. Continue to simmer uncovered for 3 to 5 minutes, stirring frequently, until the mixture thickens slightly and becomes syrupy. Take care to avoid a rapid boil or scorching.

Remove the saucepan from the heat, wait 30 seconds, then stir in ½ teaspoon vanilla extract.

Immediately pour the hot butter-sugar mixture evenly over the arranged saltine crackers. Use a spatula to spread the mixture so most crackers are coated.

Bake the pan in the preheated 350°F oven for 5 to 7 minutes, or until the toffee mixture is bubbling across the surface.

Remove the pan from the oven and let it rest on a heatproof surface for 3 to 5 minutes.

Sprinkle 1½ to 2 cups semi-sweet chocolate chips evenly over the hot toffee. Wait 3 to 5 minutes for the chips to soften and begin to melt.

Use an offset spatula or the back of a spoon to spread the melted chocolate into an even layer over the toffee.

Allow the pan to cool completely at room temperature until the chocolate and toffee are set. To speed this, refrigerate or freeze until hardened. Once firm, lift the foil out of the pan and break into pieces of your desired size.

Store pieces in an airtight container at room temperature for up to 2 weeks, or in the freezer for up to 4 months.