Preheat your oven to 400F.
Line a rimmed baking sheet or jelly roll pan (10x15 is the most ideal for the saltine crackers to fit better but a half sheet baking pan works too) with parchment paper, foil, or a silicone baking mat.
Line up the saltine crackers in a single layer on the prepared baking sheet, covering the entire surface. Feel free to break a few crackers to fill in any gaps, depending on the baking sheet you end up using.
In a saucepan over medium heat, add the butter and brown sugar. Stir constantly until the butter melts and the mixture starts to bubble. Once the mixture begins to boil, set a timer for 2 minutes. After 2 minutes is up, stir in the fish sauce until it's combined and cook for an additional 2 minutes.
Immediately pour it over the saltine crackers. Working quickly, use an offset spatula to spread the toffee mixture evenly over the crackers. Bake in the oven for 5 minutes until the toffee mixture starts to bubble. Remove the crackers from the oven and turn the oven off.
Sprinkle the chocolate chips over the crackers and place it back in the oven, with the heat off. The residual heat will melt the chocolate chips without overheating it. Leave it in the oven for 2-3 minutes until the chips started to look glossy. Spread the chocolate evenly around the crackers and top with flakey sea salt, as desired. Allow it to cool completely before breaking it into pieces, about 1-2 hours.