Preheat your oven to 375°F (190°C).
Line a 10x15 inch or 11x17 inch rimmed baking sheet with parchment paper or heavy-duty aluminum foil, ensuring the edges are well-covered.
Arrange the saltine crackers in a single layer on the prepared baking sheet, fitting them as closely together as possible.
In a medium saucepan over medium-high heat, combine the unsalted butter and packed dark brown sugar.
Bring the mixture to a full rolling boil. Once boiling, cook for exactly 3 minutes without stirring to ensure the toffee sets correctly.
Immediately pour the hot toffee mixture over the crackers, spreading it quickly with a heat-proof spatula to cover all the crackers completely.
Place the baking sheet in the oven and bake for 5 minutes, or until the toffee is bubbling vigorously across the entire pan.
Remove from the oven and immediately sprinkle the semi-sweet chocolate chips evenly over the hot toffee layer.
Let the chocolate chips sit for 5 minutes to soften from the residual heat, then use an offset spatula to spread the chocolate into a smooth, even layer.
While the chocolate is still wet, sprinkle the chopped pecans and flaky sea salt over the top.
Allow the pan to cool to room temperature, then place it in the refrigerator for at least 2 hours (or the freezer for 1 hour) until the chocolate and toffee are completely firm.
Once set, lift the parchment paper or foil out of the pan and break the candy into irregular pieces or cut into uniform squares using a sharp knife.