Preheat your oven to 375°F (190°C). Line a 10x15-inch rimmed baking sheet with parchment paper, making sure it covers the bottom and sides.
Arrange the saltine crackers in a single, tight layer on the prepared baking sheet, covering the entire surface.
In a medium saucepan, melt the butter and brown sugar together over medium heat. Bring the mixture to a rolling boil.
Once boiling, cook for exactly 3 minutes, stirring constantly with a whisk or spatula to prevent it from burning.
Immediately and carefully pour the hot toffee mixture evenly over the saltine crackers. Use a spatula to quickly spread it to the edges.
Bake for 5 to 6 minutes, until the toffee layer is bubbly across the entire surface.
Remove the pan from the oven and place it on a heat-safe surface. Sprinkle the chocolate chips evenly over the hot toffee.
Let the pan stand for 2 to 3 minutes to allow the chocolate chips to soften and melt. Use an offset spatula to spread the melted chocolate into a smooth, even layer.
If using, immediately top with flaky sea salt or sprinkles. Let the toffee cool completely at room temperature for about 2 hours, or in the refrigerator for about 1 hour, until firm.
Once hardened, lift the parchment paper to remove the toffee from the pan. Break it into irregular pieces by hand or cut into squares with a sharp knife.